[InstCombine] canonicalize fmin/fmax to LLVM intrinsics minnum/maxnum

This transform came up in D62414, but we should deal with it first.
We have LLVM intrinsics that correspond exactly to libm calls (unlike
most libm calls, these libm calls never set errno).
This holds without any fast-math-flags, so we should always canonicalize
to those intrinsics directly for better optimization.
Currently, we convert to fcmp+select only when we have FMF (nnan) because
fcmp+select does not preserve the semantics of the call in the general case.

Differential Revision: https://reviews.llvm.org/D63214

llvm-svn: 364714

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