[mips] Add instruction alias (sll and srl).

Summary:
The pattern sll/srl $rd, $rt, $rs is found in handwritten assembly which
is just a shorthand version of sllv/srlv $rd, $rt, $rs.

Reviewers: dsanders

Reviewed By: dsanders

Differential Revision: http://reviews.llvm.org/D3483

llvm-svn: 207657