[mips] Marked the ADDi instruction aliases as not available in Mips32R6 and Mips64R6.

Patch by Vasileios Kalintiris.

Differential Revision: http://reviews.llvm.org/D5242



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217867 91177308-0d34-0410-b5e6-96231b3b80d8