Function can_use_precomputed_hash_indexing_array

* If array is indexed by a constant string like in $a['somekey'], we want to precompute * the string key hash at the compile time so we don't have to do it during the run time. * This method checks whether a key is a constant key and if it is, computes the hash. * @return int string_hash or 0 (if string hash itself is 0, this array access won't be optimized) */