[mips][msa] Added partial support for matching fmax_a from normal IR (i.e. not intrinsics)

This covers the case where fmax_a can be used to implement ISD::FABS.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@191296 91177308-0d34-0410-b5e6-96231b3b80d8