[PowerPC] Add a TableGen relation for A-type and M-type VSX FMA instructions

TableGen will create a lookup table for the A-type FMA instructions providing
their corresponding M-form opcodes. This will be used by upcoming commits.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204746 91177308-0d34-0410-b5e6-96231b3b80d8