checker: simplify for smartcast in for_stmt() (#13619)

Author: yuyi98

vlib/v/checker/for.v

@@ -150,21 +150,11 @@ fn (mut c Checker) for_stmt(mut node ast.ForStmt) {
 		c.error('non-bool used as for condition', node.pos)
 	}
 	if mut node.cond is ast.InfixExpr {
-		infix := node.cond
-		if infix.op == .key_is {
-			if infix.right is ast.TypeNode && infix.left in [ast.Ident, ast.SelectorExpr] {
-				is_variable := if mut infix.left is ast.Ident {
-					infix.left.kind == .variable
-				} else {
-					true
-				}
-				left_type := c.expr(infix.left)
-				left_sym := c.table.sym(left_type)
-				if is_variable {
-					if left_sym.kind in [.sum_type, .interface_] {
-						c.smartcast(infix.left, infix.left_type, infix.right.typ, mut
-							node.scope)
-					}
+		if node.cond.op == .key_is {
+			if node.cond.right is ast.TypeNode && node.cond.left in [ast.Ident, ast.SelectorExpr] {
+				if c.table.type_kind(node.cond.left_type) in [.sum_type, .interface_] {
+					c.smartcast(node.cond.left, node.cond.left_type, node.cond.right_type, mut
+						node.scope)
 				}
 			}
 		}