[VPlan] Move VF and UF string generation to getName() (NFC).

The VFs and UFs may be more constrained as the plans are transformed
(e.g. see D135017 for an example).

To make sure the VFs/UFs included in the VPlan dump are accurate,
generate them when accessing a plan's name, rather than include them in
the name string set after initial construction.

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -9130,18 +9130,10 @@ VPlanPtr LoopVectorizationPlanner::buildVPlanWithVPRecipes(
       }
   }
 
-  std::string PlanName;
-  raw_string_ostream RSO(PlanName);
-  ElementCount VF = Range.Start;
-  Plan->addVF(VF);
-  RSO << "Initial VPlan for VF={" << VF;
-  for (VF *= 2; ElementCount::isKnownLT(VF, Range.End); VF *= 2) {
+  for (ElementCount VF = Range.Start; ElementCount::isKnownLT(VF, Range.End);
+       VF *= 2)
     Plan->addVF(VF);
-    RSO << "," << VF;
-  }
-  RSO << "},UF>=1";
-  RSO.flush();
-  Plan->setName(PlanName);
+  Plan->setName("Initial VPlan");
 
   // From this point onwards, VPlan-to-VPlan transformations may change the plan
   // in ways that accessing values using original IR values is incorrect.

llvm/lib/Transforms/Vectorize/VPlan.cpp

@@ -757,7 +757,7 @@ LLVM_DUMP_METHOD
 void VPlan::print(raw_ostream &O) const {
   VPSlotTracker SlotTracker(this);
 
-  O << "VPlan '" << Name << "' {";
+  O << "VPlan '" << getName() << "' {";
 
   if (VectorTripCount.getNumUsers() > 0) {
     O << "\nLive-in ";
@@ -789,6 +789,20 @@ void VPlan::print(raw_ostream &O) const {
   O << "}\n";
 }
 
+std::string VPlan::getName() const {
+  std::string Out;
+  raw_string_ostream RSO(Out);
+  RSO << Name;
+  if (!VFs.empty()) {
+    RSO << " for VF={" << VFs[0];
+    for (ElementCount VF : drop_begin(VFs))
+      RSO << "," << VF;
+    RSO << "},UF>=1";
+  }
+
+  return Out;
+}
+
 LLVM_DUMP_METHOD
 void VPlan::printDOT(raw_ostream &O) const {
   VPlanPrinter Printer(O, *this);

llvm/lib/Transforms/Vectorize/VPlan.h

@@ -2601,7 +2601,8 @@ class VPlan {
 
   bool hasScalarVFOnly() const { return VFs.size() == 1 && VFs[0].isScalar(); }
 
-  const std::string &getName() const { return Name; }
+  /// Return a string with the name of the plan and the applicable VFs and UFs.
+  std::string getName() const;
 
   void setName(const Twine &newName) { Name = newName.str(); }