It's MUCH easier with intrinsics. Teaching it to understand assembly involves learning a foreign language, and also for the _compiler_ to understand and predict the product of the codegen step. The compiler is usually completely separated from the codegen, it doesn't understand the architecture it targets. But using the knowledge supplied by the intrinsic API, it can do the optimisations it needs to in the usual way.

Declare the intrinsic as pure/nothrow, declare its arguments as in/scope/const/etc, it now knows a lot more about what to expect from the magic code beneath the intrinsic, and it can safely perform regular optimisation around it. Also, the codegen can use standard register assignment, which is important, and integrate nicely with regular program control structure.