[GlobalISel] Simplify G_ADD when it has (0-X) on the LHS or RHS

This implements the following combines:

((0-A) + B) -> B-A
(A + (0-B)) -> A-B

Porting over the basic algebraic combines from the DAGCombiner. There are
several combines which fold adds away into subtracts. This is just the simplest
one.

I noticed that add combines are some of the most commonly hit across CTMark,
(via print statements when they fire), so I'm porting over some of the obvious
ones.

This gives some minor code size improvements on CTMark at -O3 on AArch64.

Differential Revision: https://reviews.llvm.org/D77453