[VPlan] Make start operand non-optional for VPHeaderPHIRecipe (NFC).

The start value is always available, require it unconditionally.

Author: fhahn

llvm/lib/Transforms/Vectorize/VPlan.h

@@ -1641,10 +1641,8 @@ class VPVectorPointerRecipe : public VPRecipeWithIRFlags,
 class VPHeaderPHIRecipe : public VPSingleDefRecipe {
 protected:
   VPHeaderPHIRecipe(unsigned char VPDefID, Instruction *UnderlyingInstr,
-                    VPValue *Start = nullptr, DebugLoc DL = {})
-      : VPSingleDefRecipe(VPDefID, ArrayRef<VPValue *>(), UnderlyingInstr, DL) {
-    if (Start)
-      addOperand(Start);
+                    VPValue *Start, DebugLoc DL = {})
+      : VPSingleDefRecipe(VPDefID, ArrayRef<VPValue *>({Start}), UnderlyingInstr, DL) {
   }
 
 public: