G1: Improve G1BarrierSet::enqueue performance

While looking at startup/warmup benchmarks for VarHandles, I noticed that G1BarrierSet::enqueue is not inlined and quite hot. Its uses also load the oop first, and then check if queues are active. This could be improved a bit.

This only matters for the barrier paths that VM takes, which is the case for java.lang.invoke VM infra.