[mips] Correct itin class for MULT_MM and MULTu_MM to IIImult.

This matches the itin class used by the non-microMIPS equivalents of these
instructions.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199389 91177308-0d34-0410-b5e6-96231b3b80d8