[llvm] [LoopUtils] Cache VFs in addDiffRuntimeChecks (NFC) (PR #130157)

[David Sherwood via llvm-commits]

================
@@ -2048,12 +2048,18 @@ Value *llvm::addDiffRuntimeChecks(
   // Map to keep track of created compares, The key is the pair of operands for
   // the compare, to allow detecting and re-using redundant compares.
   DenseMap<std::pair<Value *, Value *>, Value *> SeenCompares;
+  // Map to detect redundant values returned by GetVF.
+  DenseMap<Type *, Value *> SeenVFs;
   for (const auto &[SrcStart, SinkStart, AccessSize, NeedsFreeze] : Checks) {
     Type *Ty = SinkStart->getType();
+    Value *VF = SeenVFs.lookup(Ty);
----------------
david-arm wrote:

This looks like a bug fix to me, given we were previously using the same VF value regardless of the type of SinkStart. Although I suppose it's hard to see how the bug would trigger. Even in scenarios like this:

1. Diff1 = Some value, Ty = Some type
2. Diff2 = Diff1, Ty = Different type

Then previously for 2 we'd think we'd already seen this and move on without generating a check. But in this case I'd expect the code below to trigger an assert:

```
    IsConflict =
        ChkBuilder.CreateICmpULT(Diff, VFTimesUFTimesSize, "diff.check");
```

https://github.com/llvm/llvm-project/pull/130157