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

[Mel Chen via llvm-commits]

================
@@ -86,14 +91,18 @@ class VPValue {
 
 public:
   /// Return the underlying Value attached to this VPValue.
-  Value *getUnderlyingValue() const { return UnderlyingVal; }
+  Value *getUnderlyingValue() const {
+    return SubclassID == VPSymbolicValueSC ? nullptr : UnderlyingVal;
----------------
Mel-Chen wrote:

```suggestion
    return isSymbolic() ? nullptr : UnderlyingVal;
```

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