Consider yield expressions as having side-effects

See #1043

test/compress/issue-1043.js

@@ -0,0 +1,30 @@
+issue_1043: {
+    options = {
+        side_effects: true
+    };
+
+    input: {
+        function* range(start = 0, end = null, step = 1) {
+            if (end == null) {
+                end = start;
+                start = 0;
+            }
+
+            for (let i = start; i < end; i += step) {
+                yield i;
+            }
+        }
+    }
+
+    expect: {
+        function* range(start = 0, end = null, step = 1) {
+            if (null == end) {
+                end = start;
+                start = 0;
+            }
+
+            for (let i = start; i < end; i += step)
+                yield i;
+        }
+    }
+}