added func parseLinearIndex
checked if ident is already indexed
added gcd test
Working version of GCD test, no false negatives in dependency check

Previous code had some false negatives (i.e. it parallelized loops which it
shouldn't have been). In addition, parseLinearIndex was rewritten from scratch
and works better now (handles more cases). At last, squashed a bug in which
we didn't detect a dependency between reads and writes to an array.
added prefix scan test