[llvm] [InstCombine] Do not simplify lshr/shl arg if it is part of fshl rotate pattern. (PR #73441)

[Eli Friedman via llvm-commits]

https://github.com/efriedma-quic approved this pull request.

Changes LGTM... but I'd like to land as a series of three commits; one for the refactor (6bbc776 and the fixup), one for the new tests (b896ff5), then one for the actual behavior change.

Please push separate PRs for the refactor and the test, then I'll merge everything for you.

https://github.com/llvm/llvm-project/pull/73441