[llvm] [AMDGPU][SDAG] Initial support for ISD::PTRADD (PR #141725)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> 64-bit address arithmetic uses the [sv]_add_u64 pseudos, which is consistent
with SDAG and means that GISel now generates 64-bit adds for gfx12.

GlobalISel should not select the pseudos. We should directly implement the legality rules for 64-bit pointer arithmetic 

https://github.com/llvm/llvm-project/pull/141725