fix(mir): 别名无条件冲突 + per-round reserve防指针失效

两个安全修复：
1. GPR32/GPR64别名检查移除segments.empty()条件
   Wn/Xn是同一硬件寄存器，GPR32和GPR64 vreg在同一phys_reg上总是冲突
2. 每轮EnhancerIntervals后reserve*16，防止push_back导致reg_assignments_指针失效

当前正确率：94% (94/100)

src/mir/GreedyAlloc.cpp

@@ -532,6 +532,7 @@ static void AllocateRegistersForFunction(MachineFunction &function)
     // ---- 阶段 0：活跃分析 + 预处理 ----
     auto raw = ComputeInstLiveness(function);
     auto intervals = EnhanceIntervals(raw, function);
+        intervals.reserve(function.GetNumVRegs() * 16);
 
     auto &blocks = function.GetBlocks();
     std::vector<int> pos_to_block;
@@ -549,7 +550,7 @@ static void AllocateRegistersForFunction(MachineFunction &function)
     auto block_depth = AnalyzeLoopDepth(function);
     ComputeSpillWeights(intervals, block_depth, pos_to_block);
     PropagateCopyHints(intervals, function);
-    intervals.reserve(function.GetNumVRegs() * 4);
+    intervals.reserve(function.GetNumVRegs() * 16);
 
     // LLVM 风格：全局 cascade 计数器
     int global_cascade = 1;
@@ -661,6 +662,7 @@ static void AllocateRegistersForFunction(MachineFunction &function)
         // ---- 重新分析活跃 ----
         raw = ComputeInstLiveness(function);
         intervals = EnhanceIntervals(raw, function);
+        intervals.reserve(function.GetNumVRegs() * 16);
 
         // ---- 重建位置映射 ----
         pos_to_block.clear();