[AArch64] Support for FP FMA when -ffp-contract=fast

Currently, the machine combiner can proceed matching when -ffast-math is on.
It should also match when only -ffp-contract=fast is specified as was the
case before when DAGCombiner was doing the job.

Patch by: Abderrazek Zaafrani <a.zaafrani@samsung.com>.

Differential Revision: https://reviews.llvm.org/D24366

llvm-svn: 281649