Beginning of alloca implementation for Mips fast-isel

Summary: Begin to add various address modes; including alloca.

Test Plan: Make sure there are no regressions in test-suite at O0/02 in mips32r1/r2

Reviewers: dsanders

Reviewed By: dsanders

Subscribers: echristo, rfuhler, llvm-commits

Differential Revision: http://reviews.llvm.org/D6426

llvm-svn: 230300