[clang][dataflow][NFC] Eliminate variable only used in assertion.

[mboehme]

Summary

This can break release builds that set -Werror because they will error out on
an unused variable.

Depends On D156229

[sammccall]

(if you prefer to keep the named variable, (void)StructVal2 is also fine)

[mboehme]

Abandoning. Already fixed at head in https://github.com/llvm/llvm-project/commit/e71bae94b04391cb47680622666d448418c0d972

clang/lib/Analysis/FlowSensitive/DataflowEnvironment.cpp

MergedEnv.addToFlowCondition(
                            A.makeEquals(MergedVal, Expr1)),
                  A.makeAnd(A.makeAtomRef(Env2.getFlowConditionToken()),
                            A.makeEquals(MergedVal, Expr2))));
     return &A.makeBoolValue(MergedVal);
   }
 
   Value *MergedVal = nullptr;
   if (auto *StructVal1 = dyn_cast<StructValue>(&Val1)) {
-    auto *StructVal2 = cast<StructValue>(&Val2);
-
     // Values to be merged are always associated with the same location in
     // `LocToVal`. The location stored in `StructVal` should therefore also
     // be the same.
-    assert(&StructVal1->getAggregateLoc() == &StructVal2->getAggregateLoc());
+    assert(&StructVal1->getAggregateLoc() ==
+           &cast<StructValue>(&Val2)->getAggregateLoc());
 
     // `StructVal1` and `StructVal2` may have different properties associated
     // with them. Create a new `StructValue` without any properties so that we
     // soundly approximate both values. If a particular analysis needs to merge
     // properties, it should do so in `DataflowAnalysis::merge()`.
     MergedVal = &MergedEnv.create<StructValue>(StructVal1->getAggregateLoc());
   } else {
     MergedVal = MergedEnv.createValue(Type);