refactor: make caller-saved color preference explicit in ColorGraph Select phase

Split color selection in ColorGraph's Select phase to explicitly
distinguish caller-saved (c<19) and callee-saved (c>=19) registers,
preferring caller-saved colors. Behavior is equivalent to the previous
implementation since GP_ALLOCATABLE already lists caller-saved registers
first, but the new logic is more explicit and provides an extension
point for future callee-save optimization.

src/mir/RegAlloc.cpp

@@ -726,14 +726,24 @@ namespace mir
         }
 
         int assigned_color = -1;
+        // 收集可用颜色，区分caller-saved和callee-saved
+        std::vector<int> caller_saved_avail;
+        std::vector<int> callee_saved_avail;
         for (int c : allocatable_regs)
         {
           if (used_colors.find(c) == used_colors.end())
           {
-            assigned_color = c;
-            break;
+            if (c < 19)
+              caller_saved_avail.push_back(c);
+            else
+              callee_saved_avail.push_back(c);
           }
         }
+        // 优先选择caller-saved颜色
+        if (!caller_saved_avail.empty())
+          assigned_color = caller_saved_avail[0];
+        else if (!callee_saved_avail.empty())
+          assigned_color = callee_saved_avail[0];
 
         if (assigned_color >= 0)
         {