[llvm] [AMDGPU][SDAG] Enable ISD::PTRADD for 64-bit AS by default (PR #146076)

[Fabian Ritter via llvm-commits]

ritter-x2a wrote:

Rebase to resolve a merge conflict in `test/CodeGen/AMDGPU/identical-subrange-spill-infloop.ll`
Internal performance testing was successful, so I'll merge once the premerge CI is through.

https://github.com/llvm/llvm-project/pull/146076