htools: rework compCV so that we can use [Node] too

htools: rework compCV so that we can use [Node] too

In some cases, we have a [Node.Node] list, instead of a Node.List
structure. Instead of converting [Node.Node] to the latter, let's
rework compCV and add a compCVNodes such that we skip this
(potentially expensive) step.

This already allows a simplification in a unittest, and will simplify
consumers of Node.computeGroups.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>