[llvm] [InstCombine] Compact shuffle operands by eliminating unused elements (PR #175255)

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon requested changes to this pull request.

This needs to be moved to vectorcombine - anything but the most trivial shuffle combines need to be cost driven.

https://github.com/llvm/llvm-project/pull/175255