pp_regcomp: dopn't special-case n->1 arg folding

pp_regcomp: dopn't special-case n->1 arg folding

Currently, pp_regcomp() specially handles the case where the result of
concating all its args is a regexp. Change this so that this only occurs
if there is a single arg.

This means that if $a is an overloaded object whose concat op returns
a regexp, then /$a+/ no longer special-cases, and thus the returned regex
is now stringified and recompiled. This slight loss of efficiency in a
rare case is necessary to allow us to delay concating for as long as possible,