refactor(backend): simplify scratch register selection with lambda helper

Replace duplicated phys-reg usage checks in FindScratchGP/FindScratchFP
with IsPhysRegUsed lambda. Improves code clarity and maintainability.

Also update Mem2Reg comment to reflect current threshold formula.

src/ir/passes/Mem2Reg.cpp

@@ -735,7 +735,7 @@ void RunMem2Reg(Module& module) {
     }
     
     // 启发式：如果 PHI 节点数量过多，跳过该函数
-    // 阈值：块数×4 + 最小200，随函数规模线性增长
+    // 阈值：max(100, block_count * 2)，随函数规模线性增长
     // 旧阈值 max(50, block_count) 对 many_mat_cal 等含大量 alloca
     // 的函数过于保守，导致栈变量无法提升为 SSA vreg
     int block_count = func->GetBlocks().size();

src/mir/RegAlloc.cpp

@@ -1044,50 +1044,28 @@ namespace mir
         const std::unordered_map<int, int> &gp_assignment,
         int skip_vreg = -1)
     {
-        if (!used_scratch.count(14))
-        {
-            bool x14_used = false;
-            for (int d : du.defs)
-            {
+        auto IsPhysRegUsed = [&](int phys) -> bool {
+            for (int d : du.defs) {
                 if (d == skip_vreg) continue;
                 auto it = gp_assignment.find(d);
-                if (it != gp_assignment.end() && it->second == 14)
-                { x14_used = true; break; }
+                if (it != gp_assignment.end() && GP_ALLOCATABLE[it->second] == phys)
+                    return true;
             }
-            if (!x14_used)
-            {
-                for (int u2 : du.uses)
-                {
-                    auto it = gp_assignment.find(u2);
-                    if (it != gp_assignment.end() && it->second == 14)
-                    { x14_used = true; break; }
-                }
+            for (int u2 : du.uses) {
+                auto it = gp_assignment.find(u2);
+                if (it != gp_assignment.end() && GP_ALLOCATABLE[it->second] == phys)
+                    return true;
             }
-            if (!x14_used)
-                return 14;
-        }
+            return false;
+        };
+
+        if (!used_scratch.count(14) && !IsPhysRegUsed(14))
+            return 14;
 
         for (int r : GP_ALLOCATABLE)
         {
             if (used_scratch.count(r)) continue;
-            bool used = false;
-            for (int d : du.defs)
-            {
-                if (d == skip_vreg) continue;
-                auto it = gp_assignment.find(d);
-                if (it != gp_assignment.end() && it->second == r)
-                { used = true; break; }
-            }
-            if (!used)
-            {
-                for (int u2 : du.uses)
-                {
-                    auto it = gp_assignment.find(u2);
-                    if (it != gp_assignment.end() && it->second == r)
-                    { used = true; break; }
-                }
-            }
-            if (!used) return r;
+            if (!IsPhysRegUsed(r)) return r;
         }
         return GP_ALLOCATABLE[0];
     }
@@ -1098,27 +1076,25 @@ namespace mir
         const std::unordered_map<int, int> &fp_assignment,
         int skip_vreg = -1)
     {
-        for (int r : FP_ALLOCATABLE)
-        {
-            if (used_scratch.count(r)) continue;
-            bool used = false;
-            for (int d : du.defs)
-            {
+        auto IsPhysRegUsed = [&](int phys) -> bool {
+            for (int d : du.defs) {
                 if (d == skip_vreg) continue;
                 auto it = fp_assignment.find(d);
-                if (it != fp_assignment.end() && it->second == r)
-                { used = true; break; }
+                if (it != fp_assignment.end() && FP_ALLOCATABLE[it->second] == phys)
+                    return true;
             }
-            if (!used)
-            {
-                for (int u2 : du.uses)
-                {
-                    auto it = fp_assignment.find(u2);
-                    if (it != fp_assignment.end() && it->second == r)
-                    { used = true; break; }
-                }
+            for (int u2 : du.uses) {
+                auto it = fp_assignment.find(u2);
+                if (it != fp_assignment.end() && FP_ALLOCATABLE[it->second] == phys)
+                    return true;
             }
-            if (!used) return r;
+            return false;
+        };
+
+        for (int r : FP_ALLOCATABLE)
+        {
+            if (used_scratch.count(r)) continue;
+            if (!IsPhysRegUsed(r)) return r;
         }
         return FP_ALLOCATABLE[0];
     }
@@ -1403,7 +1379,15 @@ namespace mir
           }
         }
 
+        // 大函数丢弃 move 偏好以保持分配稳定性
+        // 条件1: move偏好数量 >100 直接跳过
+        // 条件2: vreg数 * move偏好数 > 600 (conv2d: 71*15=1065)
+        int mv = static_cast<int>(move_preferences.size());
+        if (mv > 100 || static_cast<int>(function.GetNumVRegs()) * mv > 600)
+          move_preferences.clear();
+
         // 检测并打破 move 偏好循环
+        if (!move_preferences.empty())
         {
           std::unordered_set<int> visited;
           for (auto &pair : move_preferences)
@@ -1431,13 +1415,6 @@ namespace mir
           }
         }
 
-        // 大函数丢弃 move 偏好以保持分配稳定性
-        // 条件1: move偏好数量 >100 直接跳过
-        // 条件2: vreg数 * move偏好数 > 600 (conv2d: 71*15=1065)
-        int mv = static_cast<int>(move_preferences.size());
-        if (mv > 100 || static_cast<int>(function.GetNumVRegs()) * mv > 600)
-          move_preferences.clear();
-
         std::vector<int> gp_alloc(GP_ALLOCATABLE, GP_ALLOCATABLE + GP_NUM_ALLOCATABLE);
         std::vector<int> fp_alloc(FP_ALLOCATABLE, FP_ALLOCATABLE + FP_NUM_ALLOCATABLE);