[llvm] [PatternMatch] Refactor `m_SpecificInt` to avoid constructing APInt. NFC. (PR #86259)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

Should this be done for SDPatternMatch as well? I don't think MIPatternMatch has an equivalent though

https://github.com/llvm/llvm-project/pull/86259