[IRBuilder] Fix CreateFDivFMF ignoring source FMF

This change fixes a small bug in IRBuilderBase::CreateFDivFMF introduced
by accident in rGf34dcf27637f which caused the builder's default fast math
flags to be used for the new fdiv instruction instead of the ones from
the source instruction. It also adds unit testing for the CreateF*FMF
family of functions to more easily catch similar bugs in the future.

Differential Revision: https://reviews.llvm.org/D139583