[hir] Add test case for incorrect memoisation of mutated objects

In this case, x should not be cached within the if or else blocks as it is 
mutated later.

compiler/forget/src/__tests__/fixtures/hir/obj-mutated-after-if-else.expect.md

@@ -0,0 +1,65 @@
+
+## Input
+
+```javascript
+function foo(a, b, c, d) {
+  let x = someObj();
+  if (a) {
+    x = someObj();
+  } else {
+    x = someObj();
+  }
+
+  x.f = 1;
+  return x;
+}
+
+```
+
+## Code
+
+```javascript
+function foo(a, b, c, d) {
+  const $ = React.useMemoCache();
+  const x = someObj();
+  const c_0 = $[0] !== a;
+  let x$0;
+  if (c_0) {
+    x$0 = undefined;
+
+    if (a) {
+      let x$1;
+
+      if ($[2] === Symbol.for("react.memo_cache_sentinel")) {
+        x$1 = someObj();
+        $[2] = x$1;
+      } else {
+        x$1 = $[2];
+      }
+
+      x$0 = x$1;
+    } else {
+      let x$2;
+
+      if ($[3] === Symbol.for("react.memo_cache_sentinel")) {
+        x$2 = someObj();
+        $[3] = x$2;
+      } else {
+        x$2 = $[3];
+      }
+
+      x$0 = x$2;
+    }
+
+    x$0.f = 1;
+    $[0] = a;
+    $[1] = x$0;
+  } else {
+    x$0 = $[1];
+  }
+
+  return x$0;
+}
+
+```
+      

compiler/forget/src/__tests__/fixtures/hir/obj-mutated-after-if-else.js

@@ -0,0 +1,11 @@
+function foo(a, b, c, d) {
+  let x = someObj();
+  if (a) {
+    x = someObj();
+  } else {
+    x = someObj();
+  }
+
+  x.f = 1;
+  return x;
+}