[AMDGPU][NFC] Update a comment about FLAT v/s LDSDMA

The change in #170263 does not do justice to common knowledge in the backend.
Fix the comment to reflect the relation between FLAT encoding, flat pointer
access, and LDSDMA operations.

Author: ssahasra

llvm/lib/Target/AMDGPU/SIInsertWaitcnts.cpp

@@ -2297,12 +2297,11 @@ void SIInsertWaitcnts::updateEventWaitcntAfter(MachineInstr &Inst,
       ScoreBrackets->updateByEvent(LDS_ACCESS, Inst);
     }
 
-    // If this is a truly flat memory operation, then it accesss both VMEM and
-    // LDS, so note it - it will require that both the VM and LGKM be flushed to
-    // zero if it is pending when a VM or LGKM dependency occurs.
-    //
-    // For example, LDS DMA operations have FLAT set in their TSFlags for
-    // unspecified reasons, but they are not flat operations)
+    // Async/LDSDMA operations have FLAT encoding but do not actually use flat
+    // pointers. They do have two operands that each access global and LDS, thus
+    // making it appear at this point that they are using a flat pointer. Filter
+    // them out, and for the rest, generate a dependency on flat pointers so
+    // that both VM and LGKM counters are flushed.
     if (!SIInstrInfo::isLDSDMA(Inst) && FlatASCount > 1)
       ScoreBrackets->setPendingFlat();
   } else if (SIInstrInfo::isVMEM(Inst) &&