[mips] Add instruction alias (sltu).

Summary:
The pattern sltu $r1, $r2, $imm is found in handwritten assembly which
is just a shorthand version of sltui $r1, $r2, $imm.

Reviewers: dsanders

Reviewed By: dsanders

Differential Revision: http://reviews.llvm.org/D3508

llvm-svn: 207671