[llvm] [VectorCombine] foldShuffleOfShuffles - fold shuffle(shuffle(x,y),poison) length changing masks (PR #144690)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

@davemgreen Just to be clear - this patch doesn't introduce handling of length changing shuffles to foldShuffleOfShuffles - it just fixes a limitation when addressing a shuffle of a single inner shuffle.

https://github.com/llvm/llvm-project/pull/144690