[FPEnv][FMF] Move helper function to header, move fast math flags to new include file.

In a prior review I was asked to move the helper function canIgnoreSNaN()
out to FPEnv.h. This wasn't possible at the time because that function
needs the fast math flags, and including them includes lots of other stuff
that isn't needed.

This patch moves the fast math flags out into a new FMF.h file unchanged,
and moves the helper function out to FPEnv.h also unchanged. This ticket
only moves code around.

Differential Revision: https://reviews.llvm.org/D119752