Add bulk of returning of values to Mips fast-isel

Summary:
Implement the bulk of returning values in Mips fast-isel



Test Plan:
reatabi.ll

Passes test-suite at -O0,-O2 and with mips32r2 and mips32r1.





Reviewers: dsanders

Reviewed By: dsanders

Subscribers: llvm-commits, aemerson, rfuhler

Differential Revision: http://reviews.llvm.org/D5920

llvm-svn: 228958