[VPlan] Remove getPreheader(). (NFC)

The preheader is now the entry block, connected to the vector.ph.

Clean up after #114292.

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -7778,12 +7778,12 @@ DenseMap<const SCEV *, Value *> LoopVectorizationPlanner::executePlan(
   assert(DT->verify(DominatorTree::VerificationLevel::Fast));
 #endif
 
-  // 0. Generate SCEV-dependent code into the preheader, including TripCount,
-  // before making any changes to the CFG.
-  if (!BestVPlan.getPreheader()->empty()) {
+  // 0. Generate SCEV-dependent code in the entry, including TripCount, before
+  // making any changes to the CFG.
+  if (!BestVPlan.getEntry()->empty()) {
     State.CFG.PrevBB = OrigLoop->getLoopPreheader();
     State.Builder.SetInsertPoint(OrigLoop->getLoopPreheader()->getTerminator());
-    BestVPlan.getPreheader()->execute(&State);
+    BestVPlan.getEntry()->execute(&State);
   }
   if (!ILV.getTripCount())
     ILV.setTripCount(State.get(BestVPlan.getTripCount(), VPLane(0)));
@@ -10049,7 +10049,7 @@ preparePlanForEpilogueVectorLoop(VPlan &Plan, Loop *L,
   // TODO: This is a workaround needed for epilogue vectorization and it
   // should be removed once induction resume value creation is done
   // directly in VPlan.
-  for (auto &R : make_early_inc_range(*Plan.getPreheader())) {
+  for (auto &R : make_early_inc_range(*Plan.getEntry())) {
     auto *ExpandR = dyn_cast<VPExpandSCEVRecipe>(&R);
     if (!ExpandR)
       continue;

llvm/lib/Transforms/Vectorize/VPlan.h

@@ -4048,12 +4048,6 @@ class VPlan {
     SCEVToExpansion[S] = V;
   }
 
-  /// \return The block corresponding to the original preheader.
-  /// FIXME: There's no separate preheader any longer and Entry now serves the
-  /// same purpose as the original preheader. Remove after transition.
-  VPBasicBlock *getPreheader() { return Entry; }
-  const VPBasicBlock *getPreheader() const { return Entry; }
-
   /// Clone the current VPlan, update all VPValues of the new VPlan and cloned
   /// recipes to refer to the clones, and return it.
   VPlan *duplicate();