[mips][mips64r6] addi is not available on MIPS32r6/MIPS64r6

Summary: Depends on D3787. Tablegen will raise an assertion without it.

Reviewers: zoran.jovanovic, jkolek, vmedic

Reviewed By: vmedic

Differential Revision: http://reviews.llvm.org/D3842

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209419 91177308-0d34-0410-b5e6-96231b3b80d8