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

[Simon Pilgrim via llvm-commits]

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RKSimon wrote:

```suggestion
    if (ConstantFPSDNode *C = isConstOrConstSplatFP(V, /*AllowUndefs=*/true))
```

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