[llvm] InstSimplify: lookthru casts, binops in folding shuffles (PR #92668)

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

> You shouldn't be creating new instructions in instsimplify. It's for returning operands or existing instructions

Okay, abandoning this PR, as the task is impossible to do within instsimplify. I'll move the tests to VectorCombine, and work with @davemgreen on his code.

https://github.com/llvm/llvm-project/pull/92668