[llvm] [InstCombine] Pull extract through broadcast (PR #143380)

[via llvm-commits]

agorenstein-nvidia wrote:

Thanks for the feedback on how to take this further, @nikic and @RKSimon ! I've just pushed some changes with the intention of realizing those extensions. I've added additional tests, including a couple negative cases (inspired by RKSimon's mention of `undef`).

I'll add that we can assume `idx` is valid (or at least, not-known-to-be-invalid) by earlier checks in the same instcombine routine, so this shouldn't be over-aggressive in removing `poison` opportunities when replacing `idx` with `mask[0]` (so-to-speak).

https://github.com/llvm/llvm-project/pull/143380