[llvm] [AMDGPU] Fix maximum physical LDS reporting for gfx6 (PR #223185)

[Shilei Tian via llvm-commits]

shiltian wrote:

um...honestly I think LLVM source code is more convincing, especially after this many years. Lol. We might indeed overlook something in the source code though, so I'll leave this to @arsenm.

https://github.com/llvm/llvm-project/pull/223185