Fix order of evaluation for unused binary operators

eg

  (new X) == (new X);

Was converted to something like:

  (new X);
  (new X);

Which calls the destructor for the first X before constructing the second.

I tried a fix where we used an ExpressionList with ListKindLeft, which
would preserve both expressions until after both objects are created;
but that ends up calling the second object's destructor first. Thats
much better; its not clear to me that there's any guarantee about which
object's destructor is called first when they both go out of scope at
the same point; but currently hhvm and zend seem to aggree, so Im
going with a solution that preserves the left-to-right order.

hphp/compiler/expression/binary_op_expression.cpp

@@ -163,12 +163,16 @@ bool BinaryOpExpression::isLogicalOrOperator() const {
 }
 
 ExpressionPtr BinaryOpExpression::unneededHelper() {
-  if (!isShortCircuitOperator() || !m_exp2->getContainedEffects()) {
+  bool shortCircuit = isShortCircuitOperator();
+  if (!m_exp2->getContainedEffects() ||
+      (!shortCircuit && !m_exp1->getContainedEffects())) {
     return Expression::unneededHelper();
   }
 
-  m_exp2 = m_exp2->unneeded();
-  m_exp2->setExpectedType(Type::Boolean);
+  if (shortCircuit) {
+    m_exp2 = m_exp2->unneeded();
+    m_exp2->setExpectedType(Type::Boolean);
+  }
   return static_pointer_cast<Expression>(shared_from_this());
 }