According to v-spec 1.0, vmulh, vmulhu, vmulhsu and vsmul are
NOT supported for EEW=64 in Zve64*.
This patch tries to guard it correctly.
Authored by: Craig Topper <craig.topper@sifive.com> @craig.topper
Co-Authored by: Eop Chen <eop.chen@sifive.com> @eopXD
Can you add a comment here. I should have in my original patch.