make SvREFCNT_dec() more efficient

make SvREFCNT_dec() more efficient

Historically, SvREFCNT_dec was just

    #define SvREFCNT_dec(sv)        sv_free((SV*)(sv))

then in 5.10.0, for GCC, the macro was partially inlined, avoiding a
function call for the refcnt > 1 case. Recently, the macro was turned into
an inline function, providing the function-call avoidance to other
platforms too. However, the macro/inline-function is quite big, and
appears over 500 times in the core source. Its action is logically
equivalent to:

    if (sv) {
        if (SvREFCNT(sv) > 1)
            SvREFCNT(sv)--;
        else if (SvREFCNT == 1) {
            // normal case
            SvREFCNT(sv)--;
            sv_free2(sv);
        }
        else {
            // exceptional case
            sv_free(sv);
        }
    }

Where sv_free2() handles the "normal" quick cases, while sv_free()
handles the odd cases (e,g. a ref count already at 0 during global
destruction).

This means we have to plant code that potentially calls two different
subs, over 500 times.

This commit changes SvREFCNT_dec and sv_free2() to look like:

    PERL_STATIC_INLINE void
    S_SvREFCNT_dec(pTHX_ SV *sv)
    {
        if (sv) {
            U32 rc = SvREFCNT(sv);
            if (rc > 1)
                SvREFCNT(sv) = rc - 1;
            else
                Perl_sv_free2(aTHX_ sv, rc);
        }
    }

    Perl_sv_free2(pTHX_ SV *const sv, const U32 rc)
    {
        if (rc == 1) {
	    SvREFCNT(sv) = 0;
            ... do sv_clear, del_SV etc ...
            return
        }
        /* handle exceptional rc == 0 */
        ...
    }

So for the normal cases (rc > 1, rc == 1) there is the same amount of
testing and function calls, but the second test has been moved inside
the sv_free2() function.

This makes the perl executable about 10-15K smaller, and apparently a bit
faster (modulo the fact that most benchmarks are just measuring noise).

The refcount is passed as a second arg to sv_free2(), as on platforms
that pass the first few args in registers, it saves reading sv->sv_refcnt
again.