[MIPS GlobalISel] Select add i32, i32

Add the minimal support necessary to lower a function that returns the
sum of two i32 values.
Support argument/return lowering of i32 values through registers only.
Add tablegen for regbankselect and instructionselect.

Patch by Petar Avramovic.

Differential Revision: https://reviews.llvm.org/D44304

llvm-svn: 329819