When debugging AOT code generation in Leyden repo I noticed that AOT JIT compilations are serial with -Xcomp even so we create a lot of compiler threads to speed up. It is because -Xcomp enforce blocking compilation and only one thread submits AOT compilations. Also -Xcomp triggers a lot more other, not AOT, compilations which we don't need during assembly phase.

I suggest to replace -Xcomp with -Xmixed for AOT assembly phase.

I want to push it into mainline first because in mainline we don't even do AOT compilation yet.

This should speed up AOT testing with -Xcomp.