[NFC][AsmPrinter] Rework/add comments (#145587)

- Add a comment explaining TrackedRegs cache in handleNewDebugValue
- Remove a comment which is not meaningful since
https://reviews.llvm.org/D4919
- Expand the subject of a comment to match the code it describes

Author: slinder1

llvm/lib/CodeGen/AsmPrinter/DbgEntityHistoryCalculator.cpp

@@ -374,6 +374,18 @@ static void handleNewDebugValue(InlinedEntity Var, const MachineInstr &DV,
                                 DbgValueHistoryMap &HistMap) {
   EntryIndex NewIndex;
   if (HistMap.startDbgValue(Var, DV, NewIndex)) {
+    // As we already need to iterate all LiveEntries when handling a DbgValue,
+    // we use this map to avoid a more expensive check against RegVars. There
+    // is an assert that we handle this correctly in addRegDescribedVar.
+    //
+    // In other terms, the presence in this map indicates the presence of a
+    // corresponding entry in RegVars.
+    //
+    // The bool value then tracks whether an entry is to be retained (true) or
+    // removed (false); as we end previous entries we speculatively assume they
+    // can be dropped from RegVars, but we then also visit the new entry whose
+    // set of debug register operands may overlap and "save" a reg from being
+    // dropped.
     SmallDenseMap<unsigned, bool, 4> TrackedRegs;
 
     // If we have created a new debug value entry, close all preceding
@@ -467,9 +479,6 @@ void llvm::calculateDbgEntityHistory(const MachineFunction *MF,
     for (const auto &MI : MBB) {
       if (MI.isDebugValue()) {
         assert(MI.getNumOperands() > 1 && "Invalid DBG_VALUE instruction!");
-        // Use the base variable (without any DW_OP_piece expressions)
-        // as index into History. The full variables including the
-        // piece expressions are attached to the MI.
         const DILocalVariable *RawVar = MI.getDebugVariable();
         assert(RawVar->isValidLocationForIntrinsic(MI.getDebugLoc()) &&
                "Expected inlined-at fields to agree");
@@ -493,8 +502,7 @@ void llvm::calculateDbgEntityHistory(const MachineFunction *MF,
       if (MI.isMetaInstruction())
         continue;
 
-      // Not a DBG_VALUE instruction. It may clobber registers which describe
-      // some variables.
+      // Other instructions may clobber registers which describe some variables.
       for (const MachineOperand &MO : MI.operands()) {
         if (MO.isReg() && MO.isDef() && MO.getReg()) {
           // Ignore call instructions that claim to clobber SP. The AArch64