With this patch we switch from the temporary dummy seeds to actual seeds
provided by the seed collector.
The seeds get sliced and each slice is used as the starting point for
vectorization.
This patch implements packing of scalar operands when the vectorizer
decides to stop vectorizing. Packing is implemented with a sequence of
InsertElement instructions.
Packing vectors requires different instructions so it's implemented in a
follow-up patch.
This patch adds support for re-scheduling already scheduled
instructions. For now this will clear and rebuild the DAG, and will
reschedule the code using the new DAG.
