[llvm] [GIsel] Add combine (sub a, (mul x, C)) -> (add a, (mul x, -C)) (PR #194282)

[Alex MacLean via llvm-commits]

AlexMaclean wrote:

> End to end IR tests are better than MIR, especially for optimizations

Sounds good! I've switched to an LL test. 

https://github.com/llvm/llvm-project/pull/194282