[llvm] [X86] Improve helper for simplifying demanded bits of compares (PR #84360)

[via llvm-commits]

goldsteinn wrote:

This is split from: https://github.com/llvm/llvm-project/pull/82290

The reason this one comes second, is that we aren't really able to test the `cmpp` simplifications without #82290 as we never get the necessary FP flags without it.

The plan is to get #82290 in, then rebase this and submit it to clean up the regressions that #82290 causes without it.

https://github.com/llvm/llvm-project/pull/84360