[llvm] [PatternMatching] Add generic API for matching constants using custom conditions (PR #85676)

[via llvm-commits]

goldsteinn wrote:

> I'd like to defer this until after #88217, which would address my primary concern with this (the ever present undef footgun).

Actually any issue getting just the default APIs (not supporting `undef`) in now? Don't imagine it will get in your way. If you think it will be a problem though, this can ofc be deferred.

https://github.com/llvm/llvm-project/pull/85676