The method `barrier_stubs_init`, currently in BarrierSetAssembler, would like to generate and store some stubs about barriers. It is not so good to place it in `BarrierSetAssembler` because the `BarrierSetAssembler` is just an assembler in order to produce machine code which is like the `MacroAssembler`. And the feature of `barrier_stubs_init` is much like `StubGenerator` which generate and store stubs. So I think it is better to place `barrier_stubs_init` in `BarrierSet`.

A bit of history:
The method `barrier_stubs_init` is firstly introduced at JDK-8166317 which only be used at SPARC & G1. And then the ZGC and Shenandoah also used it.
But currently, they (SPARC & G1, ZGC, Shenandoah) have removed or refactored the related code and don't use `barrier_stubs_init`.
The related issues:
SPARC & G1 : SPARC was removed at JDK-8241787.
ZGC: ZGC refactored the code at JDK-8230565.
Shenandoah: Shenandoah also refactored the code at JDK-8255762.