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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> > You cannot RAUW inside InstSimplify. InstSimplify is an analysis, it cannot change IR in any way.
> 
> Oh no. Looks like I have to clone the entire chain of instructions, make the replacement in the new head instruction, and return the new tail? Is InstSimplify poorly-suited for this kind of change, and should I just leave it up to VectorCombine?

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

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