[llvm] [AMDGPU][TargetLowering] Allow forming overflow op if it is legal (PR #156266)

[Jay Foad via llvm-commits]

https://github.com/jayfoad commented:

The codegen changes generally look good but I guess they are call caused by the change to shouldFormOverflowOp, and the change to expandABD is not actually being tested in this patch - right?

https://github.com/llvm/llvm-project/pull/156266