Add some missing UseShenandoahGC checks to 8u

[Aleksey Shipilev]

On 09/21/2017 02:31 PM, Roland Westrelin wrote:
> 
> http://cr.openjdk.java.net/~roland/shenandoah/8u_useshenandoahgc/webrev.00/
> 
> This adds some missing UseShenandoahGC checks so the behaviour of C2
> when shenandoah is disabled is less likely to differ from upstream 8u.

*) loopnode.cpp does not look right. With !UseShenandoahGC this predicate is always false?

  3608       if (new_ctrl != least) {

*) Not sure if guards around is_g1_wb_pre_call do the right thing against upstream. It seems that we
have added some of them for Shenandoah only? Need to see the patch against the upstream later.

Thanks,
-Aleksey