[mips][FastISel] Implement shift ops for Mips fast-isel.

Summary:
Add shift operators implementation to fast-isel for Mips.  These are shift ops
for non legal forms, i.e. i8 and i16.

Based on a patch by Reed Kotler.

Test Plan:

Reviewers: dsanders

Subscribers: echristo, rfuhler, llvm-commits

Differential Revision: http://reviews.llvm.org/D6726

llvm-svn: 235194