Note this might miss some cases where a sub-tree within an And/Or/Xor
tree is optimizeable, but not the whole tree, but in practice this seems
to work better than the alternative of keeping a set of failed nodes and
bail early.
This patch adds a heuristic to V3SplitVar, and it attempts to split up
packed variables that are only referenced via constant index,
non-overlapping bit/range selects. This can eliminate some UNOPTFLAT cases.
Geza Lore
Wilson Snyder
Geza Lore