In Perl_sv_2[inu]v_flags(), use the non-caching code whenever SvVALID() is true

In Perl_sv_2[inu]v_flags(), use the non-caching code whenever SvVALID() is true

Previous the non-caching code was only used when SvVALID() was true on a PVGV.
However, PVLVs can also perform all the roles of a PVGV, so could conceivably
be acting as FBMs. As it's safe to test SvVALID() on any scalar SV, do so, as
the compiler can combine the flag test for SvVALID() with that for
SvGMAGICAL(), producing tighter object code.