include SvREADONLY() in SvIMMORTAL() test

SvIMMORTAL() is currently defined as

      ((sv)==&PL_sv_undef || (sv)==&PL_sv_yes
    || (sv)==&PL_sv_no || (sv)==&PL_sv_placeholder)

Which is relatively slow. Some places do this:

    if (SvREADONLY(sv) && SvIMMORTAL(sv)) ...

which quickly rejects most times.

This commit simply moves the SvREADONLY test into the SvIMMORTAL macro
so that *all* uses benefit from this speedup.