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

[Adar Dagan via llvm-commits]

Adar-Dagan wrote:

@RKSimon @lukel97 Opened issue to look into implementing this transformation in VectorCombine https://github.com/llvm/llvm-project/issues/149250

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