[mips] Add instruction alias (dsll and dsrl).

Summary:
The pattern dsll/dsrl $rd, $rt, $rs is found in handwritten assembly which
is just a shorthand version of dsllv/dsrlv $rd, $rt, $rs.

Reviewers: dsanders

Reviewed By: dsanders

Differential Revision: http://reviews.llvm.org/D3486

llvm-svn: 207664