pp_regcomp: split overloading and concat tasks

pp_regcomp: split overloading and concat tasks

Make two passes through the list of args: first to apply
magic/overloading, and secondly to concatenate them. This will
shortly allow us to pass a processed, but unconcatenated list to
re_op_compile().

Also, simplify the code by treating the 1-arg case as an arg list
of length 1.  This also allows us to use the tryAMAGICregexp macro
in only one place, and thus to unroll and eliminate it.