ast/simplify: Use clone_at_zero() for "at_zero" evaluations

The correct way of using the 'at_zero' regime of simplify is to perform
the simplification on a cloned AST subtree, otherwise the "at_zero"
evaluation seeps into the main tree.

Move the effect of the 'at_zero' flag to the cloning itself, so that
the simplify flag can be retired. We assume we can rely on id2ast in
the new clone method.

frontends/ast/ast.h

@@ -336,6 +336,10 @@ namespace AST
 		// Helper for looking up identifiers which are prefixed with the current module name
 		std::string try_pop_module_prefix() const;
 
+		// helper to clone the node with some of its subexpressions replaced with zero (this is used
+		// to evaluate widths of dynamic ranges)
+		AstNode *clone_at_zero();
+
 		// helper to print errors from simplify/genrtlil code
 		[[noreturn]] void input_error(const char *format, ...) const YS_ATTRIBUTE(format(printf, 2, 3));
 	};