[llvm] [DAG] Add SpecificFP matcher (PR #167438)

[Min-Yih Hsu via llvm-commits]

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mshockwave wrote:

yeah we can provide a `double` version of `m_SpecificFP` (which you did) but for the matcher class I prefer using APFloat as well. `SpecificInt_match` also uses APInt.

https://github.com/llvm/llvm-project/pull/167438