[GISel] Add new combines for G_ADD

Patch adds new GICombineRules for G_ADD:

G_ADD(x, G_SUB(y, x)) -> y
G_ADD(G_SUB(y, x), x) -> y

Patch additionally adds new combine tests for AArch64 target for
these new rules.

Reviewed by: paquette

Differential Revision: https://reviews.llvm.org/D87936