[InstCombine] Preserve FMF in foldLogicOfFCmps.

This patch intersects the fast math flags from the two fcmps instead
of dropping them.

I poked at this a bunch with Alive2 for nnan and ninf flags and it seemed
to check out. With the other flags it told me "Couldn't prove the
correctness of the transformation". Not sure if I should just preserve
nnan and ninf?

Reviewed By: spatel, lebedev.ri

Differential Revision: https://reviews.llvm.org/D121243