Change how node pairs are generated/used

Currently, the node pairs used for allocation are a simple [(primary,
secondary)] list of tuples, as this is how they were used before the
previous patch. However, for that patch, we use them separately per
primary node, and we have to unpack this list right after generation.

Therefore it makes sense to directly generate the list in the correct
form, and remove the split from tryAlloc. This should not be slower
than the previous patch, at least, possibly even faster.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Agata Murawska <agatamurawska@google.com>