[PatternMatch] Add new FP matchers. NFC.

This adds matchers m_NonNaN, m_NonInf, m_Finite and m_NonZeroFP as well
as generic support for binding the matched value to an APFloat.

I tried to follow the existing convention of using an FP suffix for
predicates like zero and non-zero, which could be confused with the
integer versions, but not for predicates which are clearly already
FP-specific.

Differential Revision: https://reviews.llvm.org/D89038