[VPlan] Move initial VPlan block creation to constructor. (NFC)

This sets up the initial blocks needed to initialize a VPlan directly
in the constructor. This will allow tracking of all created blocks
directly in VPlan, simplifying block deletion.

Author: fhahn

llvm/lib/Transforms/Vectorize/VPlan.cpp

@@ -821,6 +821,11 @@ void VPRegionBlock::print(raw_ostream &O, const Twine &Indent,
 }
 #endif
 
+VPlan::VPlan(Loop *L) {
+  setEntry(VPIRBasicBlock::fromBasicBlock(L->getLoopPreheader()));
+  ScalarHeader = VPIRBasicBlock::fromBasicBlock(L->getHeader());
+}
+
 VPlan::~VPlan() {
   if (Entry) {
     VPValue DummyValue;
@@ -847,19 +852,17 @@ VPlanPtr VPlan::createInitialVPlan(Type *InductionTy,
                                    PredicatedScalarEvolution &PSE,
                                    bool RequiresScalarEpilogueCheck,
                                    bool TailFolded, Loop *TheLoop) {
-  VPIRBasicBlock *Entry =
-      VPIRBasicBlock::fromBasicBlock(TheLoop->getLoopPreheader());
-  VPBasicBlock *VecPreheader = new VPBasicBlock("vector.ph");
+  auto Plan = std::make_unique<VPlan>(TheLoop);
+  VPBlockBase *ScalarHeader = Plan->getScalarHeader();
+
   // Connect entry only to vector preheader initially. Entry will also be
   // connected to the scalar preheader later, during skeleton creation when
   // runtime guards are added as needed. Note that when executing the VPlan for
   // an epilogue vector loop, the original entry block here will be replaced by
   // a new VPIRBasicBlock wrapping the entry to the epilogue vector loop after
   // generating code for the main vector loop.
-  VPBlockUtils::connectBlocks(Entry, VecPreheader);
-  VPIRBasicBlock *ScalarHeader =
-      VPIRBasicBlock::fromBasicBlock(TheLoop->getHeader());
-  auto Plan = std::make_unique<VPlan>(Entry, ScalarHeader);
+  VPBasicBlock *VecPreheader = new VPBasicBlock("vector.ph");
+  VPBlockUtils::connectBlocks(Plan->getEntry(), VecPreheader);
 
   // Create SCEV and VPValue for the trip count.
   // We use the symbolic max backedge-taken-count, which works also when

llvm/lib/Transforms/Vectorize/VPlan.h

@@ -3829,6 +3829,11 @@ class VPlan {
     TripCount = TC;
   }
 
+  /// Construct a VPlan for \p L. This will create VPIRBasicBlocks wrapping the
+  /// original preheader and scalar header of \p L, to be used as entry and
+  /// scalar header blocks of the new VPlan.
+  VPlan(Loop *L);
+
   ~VPlan();
 
   void setEntry(VPBasicBlock *VPBB) {