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

[Luke Lau via llvm-commits]

https://github.com/lukel97 approved this pull request.

LGTM, but agree with @rksimon that we should look at doing these type of transforms in VectorCombine

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