[InstCombine] change canonicalization to fabs() to use FMF on fneg

This isn't the ideal fix (use FMF on the select), but it's still an
improvement until we have better FMF propagation to selects and other
FP math operators.

I don't think there's much risk of regression from this change by
not including the FMF on the fcmp any more. The nsz/nnan FMF
should be the same on the fcmp and the fneg (fsub) because they
have the same operand.

This works around the most glaring FMF logical inconsistency cited
in PR38086:
https://bugs.llvm.org/show_bug.cgi?id=38086

llvm-svn: 362909