[VPlan] Remove unneeded getNumUsers calls in replaceAllUsesWith (NFC).

As suggested post-commit for a002271, replace unnecessary getNumUsers
calls by boolean variable to indicate if users changed. Note that this
also requires an early exit to detect the case where a value is replaced
by itself.

Author: fhahn

llvm/lib/Transforms/Vectorize/VPlan.cpp

@@ -1135,36 +1135,43 @@ void VPlanIngredient::print(raw_ostream &O) const {
 template void DomTreeBuilder::Calculate<VPDominatorTree>(VPDominatorTree &DT);
 
 void VPValue::replaceAllUsesWith(VPValue *New) {
+  if (this == New)
+    return;
   for (unsigned J = 0; J < getNumUsers();) {
     VPUser *User = Users[J];
-    unsigned NumUsers = getNumUsers();
+    bool RemovedUser = false;
     for (unsigned I = 0, E = User->getNumOperands(); I < E; ++I)
-      if (User->getOperand(I) == this)
+      if (User->getOperand(I) == this) {
         User->setOperand(I, New);
+        RemovedUser = true;
+      }
     // If a user got removed after updating the current user, the next user to
     // update will be moved to the current position, so we only need to
     // increment the index if the number of users did not change.
-    if (NumUsers == getNumUsers())
+    if (!RemovedUser)
       J++;
   }
 }
 
 void VPValue::replaceUsesWithIf(
     VPValue *New,
     llvm::function_ref<bool(VPUser &U, unsigned Idx)> ShouldReplace) {
+  if (this == New)
+    return;
   for (unsigned J = 0; J < getNumUsers();) {
     VPUser *User = Users[J];
-    unsigned NumUsers = getNumUsers();
+    bool RemovedUser = false;
     for (unsigned I = 0, E = User->getNumOperands(); I < E; ++I) {
       if (User->getOperand(I) != this || !ShouldReplace(*User, I))
         continue;
 
+      RemovedUser = true;
       User->setOperand(I, New);
     }
     // If a user got removed after updating the current user, the next user to
     // update will be moved to the current position, so we only need to
     // increment the index if the number of users did not change.
-    if (NumUsers == getNumUsers())
+    if (!RemovedUser)
       J++;
   }
 }