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

[Nikita Popov via llvm-commits]

nikic wrote:

I'd like to defer this until after https://github.com/llvm/llvm-project/pull/88217, which would address my primary concern with this (the ever present undef footgun).

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