x86: reject all invalid SAE variants

So far an SAE-only specifier was accepted for static-rounding insns,
while SAE-only insns didn't accept static rounding specifiers. If
anything it would make sense the other way around, allowing SAE-only
insns to have the (ignored) rounding mode specified individually rather
than globally via -mevexrcig=. But for now make things match the SDM.