Optimize in_array at IR gen time

Summary: Wordpress often uses in_array in this pattern:

  if (in_array($field, array('ID', 'post_parent'))) {

Transform this into:

  if (array_key_exists($field, array('ID' => null, 'post_parent' => null))) {

I'm restricting this to haystack arrays with only non-numeric string
keys. Allowing numeric keys gets complicated because the semantics of
array-indexing key comparisons are different from those of in_array's
comparisons.

Reviewed By: @markw65

Differential Revision: D1597251