Mips fast-isel - handle functions which return i8 or i6 .

Summary: Allow Mips fast-isel to handle functions which return i8/i16 signed/unsigned.

Test Plan:
Make check tests are forthcoming.
Already passes test-suite at O0/O2 for Mips 32 r1/r2

Reviewers: dsanders, rkotler

Subscribers: llvm-commits, rfuhler

Differential Revision: http://reviews.llvm.org/D6765

llvm-svn: 236103