ConstraintElim: assert on invalid union field (NFC) (#115898)

getContextInst currently returns an invalid union field, when it is
called with a ConditionFact, although existing callers don't do this. In
order to error out early and serve as documentation for future callers,
add an assert forbidding the behavior.

Author: artagnon

llvm/lib/Transforms/Scalar/ConstraintElimination.cpp

@@ -164,6 +164,7 @@ struct FactOrCheck {
   }
 
   Instruction *getContextInst() const {
+    assert(!isConditionFact());
     if (Ty == EntryTy::UseCheck)
       return getContextInstForUse(*U);
     return Inst;