FlattenMapper: claim everything is an integer

(for now, matches existing behavior, but is clearly incorrect)

loopy/symbolic.py

@@ -205,7 +205,9 @@ def map_resolved_function(self, expr, *args, **kwargs):
 
 
 class FlattenMapper(FlattenMapperBase, IdentityMapperMixin):
-    pass
+    # FIXME: Lies! This needs to be made precise.
+    def is_expression_integer_valued(self, expr: ExpressionT) -> bool:
+        return True
 
 
 def flatten(expr: ArithmeticOrExpressionT) -> ArithmeticOrExpressionT: