[AMDGPU] Allow potentially negative flat scratch offsets on GFX12 (#78193)

https://github.com/llvm/llvm-project/pull/70634

 has disabled use
of potentially negative scratch offsets, but we can use it on GFX12.

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Co-authored-by: Stanislav Mekhanoshin <Stanislav.Mekhanoshin@amd.com>