[llvm] [AMDGPU][LowerBufferFatPointers] Handle addrspacecast null to p7 (PR #140775)

[Krzysztof Drewniak via llvm-commits]

krzysz00 wrote:

Landing over the objections of the undef lint since this is a test that actually handles undef

https://github.com/llvm/llvm-project/pull/140775