[mips][FastISel] Implement FastMaterializeAlloca in Mips fast-isel.

Summary: Implement the method FastMaterializeAlloca in Mips fast-isel

Based on a patch by Reed Kotler.

Test Plan:
Passes test-suite at O0/O2 for mips32 r1/r2
fastalloca.ll

Reviewers: dsanders, rkotler

Subscribers: rfuhler, llvm-commits

Differential Revision: http://reviews.llvm.org/D6742

llvm-svn: 235213