[Clang][RISCV] Guard vmulh, vsmul correctly

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

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D117913