[GlobalISel] Implement some binary reassociations, G_ADD for now

- (op (op X, C1), C2) -> (op X, (op C1, C2))
- (op (op X, C1), Y) -> (op (op X, Y), C1)

Some code duplication with the G_PTR_ADD reassociations unfortunately but no
easy way to avoid it that I can see.

Differential Revision: https://reviews.llvm.org/D150230