Updating the RMW flags for HWIntrinsic nodes

At present, RMW semantics is tracked via a combination of the compiler support for VEX-encoded instructions and the NoRMWSemantics flag.

We should update this to instead be a combination of flags: RMWSemanticsSseOnly and RMWSemanticsAlways (or similar).

This will allow all scenarios to be table driven and for the check to be a positive case, rather than a negative case (which can sometimes lead to confusion).

category:implementation theme:hardware-intrinsics skill-level:intermediate cost:small