[llvm] [AMDGPULowerBufferFatPointers] Simplify and fix GEP offset emission (PR #95115)

[Nikita Popov via llvm-commits]

nikic wrote:

> I will note that addrspace(7) has the offset part being unsigned

Does that mean that doing a `gep inbounds (gep inbounds p, 1), -1` on addrspace(7) is not legal? I'd expect only the final offset to be treated as unsigned, not intermediate calculations.

https://github.com/llvm/llvm-project/pull/95115