[llvm] [llvm][CodeGen] Fix the empty interval issue in Window Scheduler(#128714) (PR #129204)

[Hua Tian via llvm-commits]

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@@ -601,8 +611,10 @@ void ModuloScheduleExpander::generateExistingPhis(
     // scheduling.
     if (NumStages == 0 && IsLast) {
       auto It = VRMap[CurStageNum].find(LoopVal);
-      if (It != VRMap[CurStageNum].end())
-        replaceRegUsesAfterLoop(Def, It->second, BB, MRI, LIS);
+      if (It != VRMap[CurStageNum].end()) {
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huaatian wrote:

I think that VRMap is a pointer to a contiguous block of memory, and VRMap[CurStageNum] does not involve hash lookup. The naming of VRMap is somewhat misleading.
![image](https://github.com/user-attachments/assets/78125b4c-4402-4ca6-8d5b-5dfa4da4a89e)


https://github.com/llvm/llvm-project/pull/129204