RFR: Check heap stability in C1 WBs

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/c1-wb-stable/webrev.01/

C2 now has the stable checks before RB-on-WB-fastpath. C1 should do the same, see the patch. This
improves Shenandoah+C1 performance around 0..3% across SPECjvm benchmarks. The only suspicious thing
for me is why the original code shape did the RB in-between the evac-check. I suspected implicit
null checks, but C1 seems to null-check explicitly before calling into MASM::shenandoah_write_barrier.

Testing: {x86_64, aarch64} builds, x86_64 hotspot_gc_shenandoah, {x86_64, aarch64} benchmarks

Thanks,
-Aleksey