[mips] Fix DSP instructions to have explicit accumulator register operands.

Check that instruction selection can select multiply-add/sub DSP instructions
from a pattern that doesn't have intrinsics.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@178406 91177308-0d34-0410-b5e6-96231b3b80d8