[InstSimplify] fold 'fcmp nnan oge X, 0.0' when X is not negative

This re-raises some of the open questions about how to apply and use fast-math-flags in IR from PR38086:
https://bugs.llvm.org/show_bug.cgi?id=38086
...but given the current implementation (no FMF on casts), this is likely the only way to predicate the 
transform.

This is part of solving PR39475:
https://bugs.llvm.org/show_bug.cgi?id=39475

Differential Revision: https://reviews.llvm.org/D53874

llvm-svn: 345725