[mips] Marked the DADDiu instruction aliases as MIPS III.

Patch by Vasileios Kalintiris.

Differential Revision: http://reviews.llvm.org/D5239



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217770 91177308-0d34-0410-b5e6-96231b3b80d8