[llvm] [AMDGPU] Improved Lowering of abs(i16) and -abs(i16) (PR #165626)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> @arsenm why SITargetLowering instead of AMDGPUTargetLowering?

R600 doesn't need this, AMDGPUTargetLowering is generally for shared stuff 

https://github.com/llvm/llvm-project/pull/165626