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

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

Funnily enough, I have started work on a VectorCombine::foldShuffleOfShuffles fold that will sort of do this (and catch many of these test cases):

 `shuffle(shuffle(x,u,m1),shuffle(y,u,m2)) -> shuffle(x,y,m3)`

But it doesn't recurse, so every fold has to be cost beneficial - are you seeing cases where we need the recursion capability?

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