[llvm] [InstCombine] Move extends across identity shuffles. (PR #146901)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

No objections, but I am concerned that we're going to eventually start finding cases where InstCombine will undo the cost driven decisions that VectorCombine has made.

https://github.com/llvm/llvm-project/pull/146901