[llvm] [AMDGPU] Disallow negative s_load offsets in isLegalAddressingMode (PR #91327)

[Jay Foad via llvm-commits]

jayfoad wrote:

This is intended to avoid some code quality regressions I saw with #89165. Perhaps it should even be folded into that PR.

https://github.com/llvm/llvm-project/pull/91327