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

[Matt Arsenault via llvm-commits]

https://github.com/arsenm approved this pull request.

Probably should be isLegalOrCustom. But also "shouldFormOverflowOp" is the kind of malformed, use context dependent hack hook we should avoid. We should try to remove this in the future

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