Remove magic literal constant in favor of named constant

For clarity only
Branch prediction hint for SvREFCNT_dec

When decrementing the refcount on an SV, we have to branch on whether or
not we've reached a refcount of 0. But a quick instrumentation shows
that in virtually any program small or large, the number of decrements
without freeing greatly outweighs the number of decrements that cause a
free. Therefore, it's a net win to suggest to the compiler to emit a
branch prediction hint to go into the just-decrement branch.