[Unity] Improved error message in ExprMutator::ReEmitBinding (#16307)

Prior to this commit, an error message would occur in
`ExprMutator::ReEmitBinding` if the struct info is missing from the
generated value.  However, because this error was generated from
inside `GetStructInfo`, it didn't include sufficient context for
debugging.  This commit checks the struct info explicitly, and
includes the context of the updated variable in the error message.

src/relax/ir/expr_functor.cc

@@ -670,7 +670,15 @@ void ExprMutator::ReEmitBinding(const VarBindingNode* binding, Expr new_value) {
     return;
   }
 
-  Var temp = WithStructInfo(new_var, GetStructInfo(new_value));
+  auto new_sinfo = new_value->struct_info_.as<StructInfo>();
+
+  ICHECK(new_sinfo)
+      << "InternalError: "
+      << "In binding of variable " << binding->var << ", the value " << new_value
+      << " does not have StructInfo.  "
+      << "This typically occurs when ReEmitBinding is called without first calling Normalize.";
+
+  Var temp = WithStructInfo(new_var, new_sinfo.value());
   if (!temp.same_as(new_var)) {
     new_var = temp;
   }