frontend: Remove register in visit_expr_is

dora-frontend/src/generator.rs

@@ -1243,6 +1243,8 @@ impl<'a> AstBytecodeGen<'a> {
         self.builder.bind_label(merge_lbl);
         self.pop_scope();
 
+        self.free_if_temp(value_reg);
+
         dest
     }
 

tests/trait/trait-object3.dora

@@ -0,0 +1,26 @@
+use std::traits::Iterator;
+
+class Range {
+    value: Int
+}
+
+impl Iterator for Range {
+    type Item = Int;
+
+    fn next(): Option[Int] {
+        if self.value <= 0 {
+            None[Int]
+        } else {
+            let result = self.value;
+            self.value -= 1;
+            Some[Int](result)
+        }
+    }
+}
+
+fn main() {
+    let r = Range(2);
+    assert(r.next() is Some(x) && x == 2);
+    assert(r.next() is Some(x) && x == 1);
+    assert(r.next() is None);
+}