feat(backend): propagate coalesced node colors in AssignColors\n\nAfter active coalescing, merged_set nodes inherit their representative's\ncolor, ensuring move-related vregs share the same physical register.

src/mir/RegAlloc.cpp

@@ -985,6 +985,18 @@ namespace mir
         }
       }
 
+      // 将合并节点的颜色设为其代表节点的颜色
+      for (int v : merged_set)
+      {
+        int rep_v = GetRep(v);
+        auto it = colored.find(rep_v);
+        if (it != colored.end())
+        {
+          colored[v] = it->second;
+          result.assignment[v] = it->second;
+        }
+      }
+
       return result;
     }