[InstSimplify] enhance fcmp fold with never-nan operand

This is another step towards correcting our usage of fast-math-flags when applied on an fcmp.
In this case, we are checking for 'nnan' on the fcmp itself rather than the operand of
the fcmp. But I'm leaving that clause in until we're more confident that we can stop
relying on fcmp's FMF.

By using the more general "isKnownNeverNaN()", we gain a simplification shown on the
tests with 'uitofp' regardless of the FMF on the fcmp (uitofp never produces a NaN).
On the tests with 'fabs', we are now relying on the FMF for the call fabs instruction
in addition to the FMF on the fcmp.

This is a continuation of D62979 / rL362879.

llvm-svn: 362903