[VPlan] Record underlying value for VPValues created by addVPValue (NFC).

[fhahn]

Summary

Now that printing VPValues uses the underlying IR value name, if
available, recording the underlying value here improves printing.

[Ayal]

Thanks for following-up quickly on https://reviews.llvm.org/D76200#inline-696071

llvm/lib/Transforms/Vectorize/VPlan.h

public:
   /// Add \p CBV to the vector of condition bit values.
   void addCBV(VPValue *CBV) {
     VPCBVs.push_back(CBV);
   }
 
   void addVPValue(Value *V) {
     assert(V && "Trying to add a null Value to VPlan");
     assert(!Value2VPValue.count(V) && "Value already exists in VPlan");
-    Value2VPValue[V] = new VPValue();
+    Value2VPValue[V] = new VPValue(V);
   }
 
   VPValue *getVPValue(Value *V) {
     assert(V && "Trying to get the VPValue of a null Value");
     assert(Value2VPValue.count(V) && "Value does not exist in VPlan");
     return Value2VPValue[V];
   }