[mips][FastISel] Implement intrinsics memset, memcopy & memmove.

[mips][FastISel] Implement intrinsics memset, memcopy & memmove.

Summary:
Implement the intrinsics memset, memcopy and memmove in MIPS FastISel.
Make some needed infrastructure fixes so that this can work.

Based on a patch by Reed Kotler.

Test Plan:
memtest1.ll
The patch passes test-suite for mips32 r1/r2 and at O0/O2

Reviewers: rkotler, dsanders

Subscribers: llvm-commits, rfuhler

Differential Revision: http://reviews.llvm.org/D7158

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@238759 91177308-0d34-0410-b5e6-96231b3b80d8