RFR: Fix C1/Shenandoah/Conc-Partial pre-barrier

[Roman Kennke]

Apparently, C1 doesn't deal well with nested control flows like this:

if (conc_mark_in_progress) {

   if (satb_active) {

     g1satbbarrierstub();

   }

}

Aleksey found a crash where C1 would sneak in what looks like a spill 
reg load after loading the satb_active field but before the actual 
branch. This one is used later (after the barrier, even if it's not 
branching out to the barrier stub). However, if we surround this by our 
own control flow, we miss that spill reg load, and the use of this 
register after the barrier makes it crash.

Turns out we don't actually need that double-checking code. We can use a 
copy of the G1 barrier code with the satb_active check swapped out by 
conc_mark_in_progress check. This fixes the crash and leads to less 
branchy code.

http://cr.openjdk.java.net/~rkennke/fixc1prebarrier/webrev.00/ 
<http://cr.openjdk.java.net/%7Erkennke/fixc1prebarrier/webrev.00/>

Test: Aleksey's failing test

Good?

Roman