[VPlan] Remove re-using vector PH in VPBasicBlock::execute (NFC).

Remove logic to re-use the previous basic block for the vector pre
header from VPBasicBlock::execute. The preheader is now modeled as
VPIRBasicBlock, so the code is no longer needed.

Split off from llvm#108378.

Author: fhahn

llvm/lib/Transforms/Vectorize/VPlan.cpp

@@ -487,11 +487,9 @@ void VPBasicBlock::execute(VPTransformState *State) {
   };
 
   // 1. Create an IR basic block.
-  if (this == getPlan()->getVectorPreheader() ||
-      (Replica && this == getParent()->getEntry()) ||
+  if ((Replica && this == getParent()->getEntry()) ||
       IsReplicateRegion(getSingleHierarchicalPredecessor())) {
     // Reuse the previous basic block if the current VPBB is either
-    //  * the vector preheader,
     //  * the entry to a replicate region, or
     //  * the exit of a replicate region.
     State->CFG.VPBB2IRBB[this] = NewBB;