Make first of several changes to bring up to AArch64 fast-isel style

Summary:
Make Mips fast-isel track the form of AArch64 where practical.
This makes it easier for people to review the code, to borrow similar code, and to see how to eventually move a lot of this
 target code for fast-isels into target independent code.

These are just cosmetic changes. Should be no functional difference.

Test Plan:
make check
test-suite for 4 flavors mips32 r1/r2 , -O0/-O2

Reviewers: dsanders

Reviewed By: dsanders

Subscribers: aemerson, llvm-commits, rfuhler

Differential Revision: http://reviews.llvm.org/D5595

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219633 91177308-0d34-0410-b5e6-96231b3b80d8