Turn on read-only flag for some unchangeable inversion lists

These lists are read-only.  Turning on the flag may allow some
optimisations to be done, including some that may be added in the
future.
regcomp.c: Remove unnecessary variable

This variable is set and tested just once in all but one instance,
before being discarded.  In that one instance, it's tested twice.  The
underlying expression is just extracting a field from a structure.
There's no efficiency gain to using the variable, so might as well not
have it.