Repro case for product bug

Simplified version of the example. As I eventually uncovered in #1342, the issue 
is cascading redundant phis not getting rewritten.

compiler/forget/src/__tests__/fixtures/hir/_bug.ssa.expect.md

@@ -0,0 +1,63 @@
+
+## Input
+
+```javascript
+function Component(props) {
+  let x = 0;
+  const values = [];
+  const y = props.a || props.b;
+  values.push(y);
+  if (props.c) {
+    x = 1;
+  }
+  values.push(x);
+  if (props.d) {
+    x = 2;
+  }
+  values.push(x);
+  return values;
+}
+
+```
+
+## Code
+
+```javascript
+function Component(props) {
+  const $ = React.unstable_useMemoCache(4);
+  const c_0 = $[0] !== props;
+  let values;
+  if (c_0) {
+    values = [];
+    const c_2 = $[2] !== props;
+    let t0;
+    if (c_2) {
+      t0 = props.a || props.b;
+      $[2] = props;
+      $[3] = t0;
+    } else {
+      t0 = $[3];
+    }
+    const y = t0;
+    values.push(y);
+    let x$0 = x;
+    if (props.c) {
+      x = 1;
+    }
+
+    values.push(x$0);
+    if (props.d) {
+      x = 2;
+    }
+
+    values.push(x);
+    $[0] = props;
+    $[1] = values;
+  } else {
+    values = $[1];
+  }
+  return values;
+}
+
+```
+      

compiler/forget/src/__tests__/fixtures/hir/_bug.ssa.js

@@ -0,0 +1,15 @@
+function Component(props) {
+  let x = 0;
+  const values = [];
+  const y = props.a || props.b;
+  values.push(y);
+  if (props.c) {
+    x = 1;
+  }
+  values.push(x);
+  if (props.d) {
+    x = 2;
+  }
+  values.push(x);
+  return values;
+}