[llvm] [AMDGPU] Update PromoteAlloca to handle GEPs with variable offset. (PR #122342)

[Matt Arsenault via llvm-commits]

================
@@ -385,16 +385,21 @@ static bool isSupportedMemset(MemSetInst *I, AllocaInst *AI,
          match(I->getOperand(2), m_SpecificInt(Size)) && !I->isVolatile();
 }
 
-static Value *
-calculateVectorIndex(Value *Ptr,
-                     const std::map<GetElementPtrInst *, Value *> &GEPIdx) {
+static Value *calculateVectorIndex(
+    Value *Ptr, const std::map<GetElementPtrInst *, WeakTrackingVH> &GEPIdx) {
   auto *GEP = dyn_cast<GetElementPtrInst>(Ptr->stripPointerCasts());
   if (!GEP)
     return ConstantInt::getNullValue(Type::getInt32Ty(Ptr->getContext()));
 
   auto I = GEPIdx.find(GEP);
   assert(I != GEPIdx.end() && "Must have entry for GEP!");
-  return I->second;
+
+  if (Value *IndexValue = I->second)
+    return IndexValue;
+
+  llvm_unreachable(
+      "Index value is not present in Cached GEPs! This indicates a "
+      "bug in the pass that populates GEPIdx.");
----------------
arsenm wrote:

```suggestion
    Value *IndexValue = I->second;
    assert(IndexValue && "index value missing from GEP index map");
    return IndexValue;
```

https://github.com/llvm/llvm-project/pull/122342