Convert Zone to multi-entry.

No significant difficulties with the conversion.
Implement loopForestSteensgaard a multi-root graph.

Simply treat each root as though there is an external edge into it.
Updates to conversion of SignalCheck to multi-entry

Commit c02befa gave an initial conversion of SignalCheck to
multi-entry.  That commit inserted signal checks at each function
entry and inserted signal checks according to the loop forest rooted
at each function entry.  The minor bug in commit c02befa is that
inserting signal checks according to the loop forest rooted at each
function entry results in repeatedly inserting signal checks and
duplicating blocks and labels.  It suffices to insert signal checks
according to the loop forest rooted at the collectio of function
entries.