[InstCombine] propagate fast-math-flags when folding fcmp+fneg

[InstCombine] propagate fast-math-flags when folding fcmp+fneg

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

This might be enough to fix that particular issue, but as noted
with the FIXME, we're still dropping FMF on other folds around here.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@346234 91177308-0d34-0410-b5e6-96231b3b80d8