[llvm] [InstCombine] Let shrinkSplatShuffle act on vectors of different lengths (PR #148593)

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon commented:

This is getting very close to needing to be cost based, which suggests we should move it the entire fold to VectorCombine - we already have VectorCombine::foldShuffleOfCastops for a shuffle of 2 matching casts, relaxing this to handle a single cast wouldn't be a huge amount of work.

https://github.com/llvm/llvm-project/pull/148593