Up regex flags limit for (??{})

Up regex flags limit for (??{})

Previously the regex pattern compilation flags needed for this construct
would fit into an 8-bit byte.  This conveniently fits into the flags
structure element of a regnode.  There are changes coming that require
more than 8 bits, so in preparation, this commit adds an argument to the
node that implements (??{}) (31-bits usable for flags), and moves the
storage to that.