[AMDGPU] Update comment on SIInstrInfo::isLegalFLATOffset for GFX12

Author: jayfoad

llvm/lib/Target/AMDGPU/SIInstrInfo.cpp

@@ -8968,8 +8968,9 @@ bool SIInstrInfo::splitMUBUFOffset(uint32_t Imm, uint32_t &SOffset,
 
 // Depending on the used address space and instructions, some immediate offsets
 // are allowed and some are not.
-// In general, flat instruction offsets can only be non-negative, global and
-// scratch instruction offsets can also be negative.
+// Pre-GFX12, flat instruction offsets can only be non-negative, global and
+// scratch instruction offsets can also be negative. On GFX12, offsets can be
+// negative for all variants.
 //
 // There are several bugs related to these offsets:
 // On gfx10.1, flat instructions that go into the global address space cannot