[llvm] [PatternMatch] Add matchers for `m_{I,F,}Cmp` and `m_{I,F,}SpecificCmp`; NFC (PR #98282)

[Nikita Popov via llvm-commits]

nikic wrote:

Hm yeah, the way the "unused predicate" case is implemented doesn't work ... the actual match will happen later, and at that point the dummy variable is out of scope. I guess we'd need a separate matcher class for that.

https://github.com/llvm/llvm-project/pull/98282