fix(backend): recompute degree unconditionally after MergeInto\n\nAfter a merge, u inherits v's neighbors, so degree[u] must always\nbe recomputed. Previously, when degree[u] < K before merge, the\nstale low degree was kept, which could push a high-degree merged\nnode into simplify_worklist with wrong metadata.\n\nAlso remove redundant if(!remaining.empty()) guard in spill path\nand clean up extra brace from removed GiveUpPhase.

src/mir/RegAlloc.cpp

@@ -765,13 +765,11 @@ namespace mir
           }
         }
         move_adj[v].clear();
-        if (degree[u] >= K)
-        {
-          int new_deg = 0;
-          for (int n : adj[u])
-            if (remaining.count(GetRep(n))) new_deg++;
-          degree[u] = new_deg;
-        }
+        // 合并后重新计算 u 的度数（邻接集已包含 v 的邻接）
+        int new_deg = 0;
+        for (int n : adj[u])
+          if (remaining.count(GetRep(n))) new_deg++;
+        degree[u] = new_deg;
         if (degree[u] < K && !HasMovePair(u))
           simplify_worklist.push_back(u);
       };
@@ -894,8 +892,6 @@ namespace mir
         }
         else
         {
-          if (!remaining.empty())
-          {
           // spill cost: len(活跃指令数)*5 + ref(def+use总次数)*15 - degree(干涉度数)*25
           // cost 越小越优先 spill —— 短区间、少引用、高冲突的变量更适合溢出
           // 权重基于经验调节：degree 项主导，len/ref 项作为 tiebreaker
@@ -950,7 +946,6 @@ namespace mir
           {
             break;
           }
-        }
       }
       }