Prevent more optimizations in repo mode

"$unused_local = UndefinedConstant;" was being optimized away, so
we didnt get a warning for the constant. return it instead.

  $unused_local = new Object;
  return;

became

  new Object;
  return;

This affected the (unspecified) order of destruction of $this
vs $unused_local. Since we're specifically trying to test that
we dont crash when a local generates a backtrace after $this
has been destroyed, force the variable to be used.

hphp/test/quick/debug_backtrace.php

@@ -29,6 +29,6 @@ function main() {
   foo();
 
   set_error_handler('error_handler');
-  $x = FakeConstant;
+  return FakeConstant;
 }
 main();

hphp/test/quick/destructed_this.php

@@ -5,6 +5,7 @@ function id($x) { return $x; }
 class Foo {
   public function hi() {
     $bad = new Tracer();
+    return $bad !== null;
   }
 }
 

hphp/test/quick/destructed_this.php.expectf

@@ -23,7 +23,7 @@ array(3) {
     ["file"]=>
     string(%d) "%s"
     ["line"]=>
-    int(18)
+    int(19)
     ["function"]=>
     string(2) "hi"
     ["class"]=>
@@ -39,7 +39,7 @@ array(3) {
     ["file"]=>
     string(%d) "%s"
     ["line"]=>
-    int(20)
+    int(21)
     ["function"]=>
     string(4) "main"
     ["args"]=>