x86-64: don't hide an empty but meaningless REX prefix

x86-64: don't hide an empty but meaningless REX prefix

Unlike for non-zero values passed to USED_REX(), where rex_used gets
updated only when the respective bit was actually set in the encoding,
zero getting passed in is not further guarded, yet such a (potentially
"empty") REX prefix takes effect only when there are registers numbered
4 and up.