[llvm] [AMDGPU] Tweak handling of GlobalAddress operands in SI_PC_ADD_REL_OFFSET (PR #70960)

[Jay Foad via llvm-commits]

jayfoad wrote:

The motivation for this is that I want to tweak the S_GETPC/S_ADD/S_ADDC expansion and I'd rather do that in one place than in two.

https://github.com/llvm/llvm-project/pull/70960