[llvm] [InstCombine] do not assume the operand position of reverse shuffle (PR #205747)

[Nikita Popov via llvm-commits]

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

LGTM

We should probably reconsider the semantics of these shufflevector queries, they are a big footgun, and I don't think the relaxed semantics are actually useful.

https://github.com/llvm/llvm-project/pull/205747