[llvm] [VectorCombine] Add foldShuffleToIdentity (PR #88693)

[Alexey Bataev via llvm-commits]

================
@@ -1547,6 +1548,148 @@ bool VectorCombine::foldShuffleOfCastops(Instruction &I) {
   return true;
 }
 
+// Starting from a shuffle, look up through operands tracking the shuffled index
+// of each lane. If we can simplify away the shuffles to identities then
+// do so.
+bool VectorCombine::foldShuffleToIdentity(Instruction &I) {
+  FixedVectorType *Ty = dyn_cast<FixedVectorType>(I.getType());
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alexey-bataev wrote:

```suggestion
  auto *Ty = dyn_cast<FixedVectorType>(I.getType());
```


https://github.com/llvm/llvm-project/pull/88693