[llvm] [VPlan] Add VPSymbolicValueSC for typed VPValues w/o underlying IR value (PR #130507)

[Mel Chen via llvm-commits]

================
@@ -61,17 +61,22 @@ class VPValue {
   SmallVector<VPUser *, 1> Users;
 
 protected:
-  // Hold the underlying Value, if any, attached to this VPValue.
-  Value *UnderlyingVal;
+  union {
+    // Hold the underlying Value, if any, attached to this non-symbolic VPValue.
+    Value *UnderlyingVal;
+    // Hold the type of this VPValue, if it is symbolic.
+    Type *Ty;
+  };
 
   /// Pointer to the VPDef that defines this VPValue. If it is nullptr, the
   /// VPValue is not defined by any recipe modeled in VPlan.
   VPDef *Def;
 
   VPValue(const unsigned char SC, Value *UV = nullptr, VPDef *Def = nullptr);
 
-  /// Create a live-in VPValue.
-  VPValue(Value *UV = nullptr) : VPValue(VPValueSC, UV, nullptr) {}
+  /// Create a live-in IR VPValue.
+  VPValue(Value *UV) : VPValue(VPValueSC, UV, nullptr) {}
+  VPValue(Type *Ty) : SubclassID(VPSymbolicValueSC), Ty(Ty), Def(nullptr) {}
----------------
Mel-Chen wrote:

Maybe directly set Def to nullptr by default?
```
VPDef *Def = nullptr;
```

https://github.com/llvm/llvm-project/pull/130507