[AMDGPU] Require explicit immediate offsets for SGPR+IMM SMEM instructions. (#79131)

As otherwise SGPR+IMM instructions are not distinguishable to SGPR-only
ones in AsmParser, leading to ambiguities.

GFX12 doesn't have special SGPR-only variants, so we still allow
optional immediate offsets for the subtarget.

Also rename the offset operand classes while there.

Part of <https://github.com/llvm/llvm-project/issues/69256>.