regcomp.c: Refactor so can remove duplicate code

regcomp.c: Refactor so can remove duplicate code

This commit prepares the way for a later commit to remove a chunk of
essentially duplicate code.  It does this at the cost of an extra
test of a boolean each time through the loop.  But, it saves calculating
the fold unless necessary, a potentially expensive operation.  When the
next input is a quantifier that calculated fold is discarded, unused.
This commit avoids doing that calculation when the next input is a
quantifier.