(cherry picked from commit 8640a6d7)

it is needed and useful in tests.

(cherry picked from commit 5f071bc3)

dependencies, and also to treat loops more carefully.

* Dependencies are now tracked based on a list of active potential
  sources, and the active set is narrowed by filtering through
  readers/writers
* Stricter handling of dependencies within loops by not assuming
  that loops are nonempty
* Changes the barrier insertion algorithm to act in two stages: first,
  barriers are recursively inserted into loops, then they are inserted
  at the outermost level. This is mostly meant to simplify the code.
* See also #18 on gitlab

insn_ids_reachable_from_start_without_intervening_barrier(): Can further narrow set of instructions by considering barriers at the shallower level.

insn_ids_reaching_*_without_interleaving_barrier().

Next try at removing the ambiguous schedule warning

See merge request !65

Better detection of dtype in TemporaryVariable initialization

See merge request !58

Barrier insertion updates

See merge request !74