[llvm] [AMDGPU] Rename hasGFX12Enc to hasRestrictedSOffset in BUF definitions. NFC. (PR #83434)

[Jay Foad via llvm-commits]

jayfoad wrote:

> I have only just noticed that there is a corresponding target feature called "hasRestrictedSOffset". Should I rename that too?

After some more discussion I settled on a simpler patch that changes the tablegen argument name to match this subtarget feature.

https://github.com/llvm/llvm-project/pull/83434