(analyse_first): New function obtained by ripping out most

(analyse_first): New function obtained by ripping out most
of re_compile_fastmap and generalizing it a little bit so that it
can also just return whether a given (sub)pattern can match the empty
string or not.
(regex_compile): Use `analyse_first' to decide whether the loop-check
needs to be done or not for *, +, *? and +? (the loop check is costly
for non-greedy repetition).
(re_compile_fastmap): Delegate the actual work to `analyse_first'.