regcomp.c: Add ability to take union of a complement

regcomp.c: Add ability to take union of a complement

Previous commits have added the ability to the inversion list
intersection routine to take the complement of one of its inputs.
Likewise, for unions, this will be a frequent paradigm, and it is
cheaper to do the complement of an input in the routine than to
construct a new temporary that is the desired complement, and throw it
away.