regex: Add lower bound to ANYOFH nodes UTF-8 byte

regex: Add lower bound to ANYOFH nodes UTF-8 byte

This commit adds a lower bound for the first UTF-8 byte matchable by an
ANYOFH node.  The flags field is otherwise unused, and using it for this
purpose allows code to rule out match possibilities without having to
convert from UTF-8 to code point.

It might be better to do the inverse instead, to have the field be an
upper bound.  The reason is that the conversion is cheap for smaller
numbers.  The commit following mostly addresses this.