Convert regnode to a flag for [...]

Convert regnode to a flag for [...]

Prior to this commit, there were 3 types of ANYOF nodes; now there are
two: regular, and one for the synthetic start class (ssc).  This commit
converted the third type dealing with warning about matching \p{}
against non-Unicode code points, into using the spare flag bit for ANYOF
nodes.

This allows this bit to apply to ssc ANYOF nodes, whereas previously it
couldn't.  There is a bug in which the warning isn't raised if the match
is rejected by the optimizer, because of this inability.  This bug will
be fixed in a later commit.

Another option would have been to create a new node-type which was an
ANYOF_SSC_WARN_SUPER node.  But this adds extra complications to things;
and we have a spare bit that we might as well use.  The comments give
better possibilities for freeing up 2 bits should they be needed.