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

[Chaitanya Koparkar via llvm-commits]

ckoparkar wrote:

> ⚠️ undef deprecator found issues in your code. ⚠️
> You can test this locally with the following command:
> ...
> In tests, avoid using `undef` and having tests that trigger undefined behavior. If you need an operand with some unimportant value, you can add a new argument to the function and use that instead.

Oops, okay I'll remove the test.

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