Deparse.pm: Put re flag logic into its own function

Deparse.pm: Put re flag logic into its own function

Also, sort the flags so that matchwords still work.  It seems they never
did work properly for s///r, so this commit causes this Freudian slip:

$ perl5.14.2 -MO=Deparse -e 's///rose'
s//();/eros;
-e syntax OK

to become this:

$ ./perl -Ilib -MO=Deparse -e 's///sore'
s//();/rose;
-e syntax OK