The regexp engine should check SV flags rather than SV type for

The regexp engine should check SV flags rather than SV type for
determining if something is a reference, because under the debugger
the value returned by the swash code is SVt_PVMG.
Not doing this has the side effect of repeatedly reassigning the
same array element, which causes destructors to fire on the reassignment
which in turn causes &utf8::DESTROY to run outside of the pseudo-safety
of save_re_context, which under the debugger involves re-entering the
regexp engine, which causes corruption of the regexp engine's global
state.

p4raw-id: //depot/perl@26228