[mips][FastISel] Handle calls with non legal types i8 and i16.

Summary: Allow calls with non legal integer types based on i8 and i16 to be processed by mips fast-isel.

Based on a patch by Reed Kotler.

Test Plan:
"Make check" test forthcoming.
Test-suite passes at O0/O2 and with mips32 r1/r2

Reviewers: rkotler, dsanders

Subscribers: llvm-commits, rfuhler

Differential Revision: http://reviews.llvm.org/D6770

llvm-svn: 237121