[llvm] [AMDGPU][SDAG] Add tests for ISD::PTRADD DAG combines (PR #142738)

[Fabian Ritter via llvm-commits]

ritter-x2a wrote:

The sanitizer buildbot fails look like unrelated flaky tests to me (especially because this PR only adds a test that has nothing to do with sanitizers).

https://github.com/llvm/llvm-project/pull/142738