[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

llvm-svn: 209419