[ARM] GlobalISel: Select add i32, i32

[ARM] GlobalISel: Select add i32, i32

Add the minimal support necessary to select a function that returns the sum of
two i32 values.

This includes some support for argument/return lowering of i32 values through
registers, as well as the handling of copy and add instructions throughout the
GlobalISel pipeline.

Differential Revision: https://reviews.llvm.org/D26677

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@289940 91177308-0d34-0410-b5e6-96231b3b80d8