make sv_2bool_flags() non-recursive on overload

When Perl_sv_2bool_flags() has an overloaded arg, it calls SvTRUE()
on the SV returned from the overload method. This indirectly calls
sv_2bool_flags() again.

Change it so that sv_2bool_flags() just iterates the new overload value
each time.

2 callsites were converted to gotos. A SvTRUE_common was expanded so goto
can be used.  This function's machine code size on VC2003 32 bits dropped
by 0x24 bytes after this patch.