[PowerPC] Correct commutable indices for VSX FMA instructions

Although the first two operands are the ones that can be swapped, the tied
input operand is listed before them, so we need to adjust for that.

I have a test case for this, but it goes along with an upcoming commit (so it
will come soon).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204748 91177308-0d34-0410-b5e6-96231b3b80d8