[llvm] [VectorCombine] foldShuffleOfCastops - handle unary shuffles (PR #160009)

[Chaitanya Koparkar via llvm-commits]

https://github.com/ckoparkar commented:

This patch is also causing some tests to timeout, looking into it:

```
LLVM :: Transforms/PhaseOrdering/X86/blendv-select.ll
LLVM :: Transforms/PhaseOrdering/X86/pr67803.ll
LLVM :: Transforms/PhaseOrdering/X86/shuffle-inseltpoison.ll
LLVM :: Transforms/PhaseOrdering/X86/shuffle.ll
LLVM :: Transforms/PhaseOrdering/X86/vec-load-combine.ll
LLVM :: Transforms/VectorCombine/AArch64/combine-shuffle-ext.ll
LLVM :: Transforms/VectorCombine/X86/shuffle-inseltpoison.ll
LLVM :: Transforms/VectorCombine/X86/shuffle.ll
```

https://github.com/llvm/llvm-project/pull/160009