[llvm] AMDGPU GlobalISel Add64 support (PR #124763)

[via llvm-commits]

tpopp wrote:

I've updated this with my understanding of your advice. Can you see if it looks correct before I add tests?

This tablegen pattern was very beneficial in the cases that I saw, but I believe the types do not cover `p0` and `s64` types for example, as the tests like https://github.com/llvm/llvm-project/blob/d246cc618adc52fdbd69d44a2a375c8af97b6106/llvm/test/CodeGen/AMDGPU/GlobalISel/inst-select-ptr-add.mir#L173 were not affected by this change.

https://github.com/llvm/llvm-project/pull/124763