[llvm] [VPlan] Speed up VPSlotTracker by using ModuleSlotTracker (PR #139881)

[Florian Hahn via llvm-commits]

================
@@ -1441,7 +1441,23 @@ void VPSlotTracker::assignName(const VPValue *V) {
   std::string Name;
   if (UV) {
     raw_string_ostream S(Name);
-    UV->printAsOperand(S, false);
+    if (MST) {
+      UV->printAsOperand(S, false, *MST);
----------------
fhahn wrote:

I was thinking if having something like

```
getNameForVPValue() {
  if (UV->hasName()) {
     UV->printAsOperand(S, false, *MST);
    return Name;
  }
  if (!MST) {
    code to init slot tracker
  }
  UV->printAsOperand(S, false, *MST);
  return Name;
}
```

https://github.com/llvm/llvm-project/pull/139881