[mips][FastISel] Allow computation of addresses from constant expressions.

Summary:
Try to compute addresses when the offset from a memory location is a constant
expression.

Based on a patch by Reed Kotler.

Test Plan:
Passes test-suite for -O0/O2 and mips 32 r1/r2

Reviewers: rkotler, dsanders

Subscribers: llvm-commits, aemerson, rfuhler

Differential Revision: http://reviews.llvm.org/D6767

llvm-svn: 237117