[llvm] [AMDGPU][SDAG] Handle ISD::PTRADD in VOP3 patterns (PR #143881)

[Fabian Ritter via llvm-commits]

ritter-x2a wrote:

@jayfoad at the moment (and within this stack of PRs), the difference does indeed not yet matter anywhere. The plan is to change that with PR #132353, which is supposed to use an `inbounds` annotation to determine that it is safe to fold immediate offsets into flat instructions.
(PR #131862 is supposed to introduce this `inbounds` annotation; the reviews there argued that `inbounds` has confusing semantics at best if it isn't attached to a `ptradd` operation (which clearly determines which of the operands is the pointer), that is what brought me to this stack of PRs here to introduce and support the `ptradd` operation.)

https://github.com/llvm/llvm-project/pull/143881