[AArch64] Check fmul node single use in fused multiply patterns

Check for single use of fmul node in fused multiply patterns
to allow generation of fused multiply add/sub instructions.
Otherwise fmul operation ends up being repeated more than
once which does not help peformance on targets with
only one MAC unit, as for example cortex-a53.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197929 91177308-0d34-0410-b5e6-96231b3b80d8