Add infinite loop test case

Discovered this by accident modifying Joe's playground example. This seemed to 
be triggered in inferReactiveScopeVariables when iterating over the DisjointSet 
of scopeIdentifiers. In particular this test case contains a cycle and 
DisjointSet.find would never terminate.

compiler/forget/src/__tests__/fixtures/hir/issue933-disjoint-set-infinite-loop.expect.md

@@ -0,0 +1,64 @@
+
+## Input
+
+```javascript
+// This causes an infinite loop in the compiler
+function MyApp(props) {
+  const y = makeObj();
+  const tmp = y.a;
+  const tmp2 = tmp.b;
+  y.push(tmp2);
+  return y;
+}
+
+```
+
+## HIR
+
+```
+bb0:
+  [1] Const mutate y$7_@0[1:5] = Call mutate makeObj$2:TFunction()
+  [2] Const mutate tmp$8_@0[1:5] = read y$7_@0.a
+  [3] Const mutate tmp2$9_@0[1:5] = read tmp$8_@0.b
+  [4] Call mutate y$7_@0.push(mutate tmp2$9_@0)
+  [5] Return freeze y$7_@0
+```
+
+## Reactive Scopes
+
+```
+function MyApp(
+  props,
+) {
+  scope @0 [1:5] deps=[] out=[y$7_@0] {
+    [1] Const mutate y$7_@0[1:5] = Call mutate makeObj$2:TFunction()
+    [2] Const mutate tmp$8_@0[1:5] = read y$7_@0.a
+    [3] Const mutate tmp2$9_@0[1:5] = read tmp$8_@0.b
+    [4] Call mutate y$7_@0.push(mutate tmp2$9_@0)
+  }
+  return freeze y$7_@0
+}
+
+```
+
+## Code
+
+```javascript
+function MyApp$0(props$6) {
+  const $ = React.useMemoCache();
+  let y$7;
+  if ($[0] === Symbol.for("react.memo_cache_sentinel")) {
+    y$7 = makeObj$2();
+    const tmp$8 = y$7.a;
+    const tmp2$9 = tmp$8.b;
+    y$7.push(tmp2$9);
+    $[0] = y$7;
+  } else {
+    y$7 = $[0];
+  }
+
+  return y$7;
+}
+
+```
+      

compiler/forget/src/__tests__/fixtures/hir/issue933-disjoint-set-infinite-loop.js

@@ -0,0 +1,8 @@
+// This caused an infinite loop in the compiler
+function MyApp(props) {
+  const y = makeObj();
+  const tmp = y.a;
+  const tmp2 = tmp.b;
+  y.push(tmp2);
+  return y;
+}