[llvm] [AMDGPU][SDAG] Add target-specific ISD::PTRADD combines (PR #143673)

[Fabian Ritter via llvm-commits]

ritter-x2a wrote:

### Merge activity

* **Jul 18, 7:54 AM UTC**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/143673).


https://github.com/llvm/llvm-project/pull/143673