G1: has_aborted() || _cm->region_stack_empty() fails

During testing we've come across this assertion failure. Poonam hit it while looking at another bug (CR 6847956).

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# Internal Error (concurrentMark.cpp:3492), pid=14287, tid=73
# Error: guarantee(has_aborted() || _cm->region_stack_empty(),"only way to exit the loop")

  [5] VMError::report_and_die(0xffffffff7e7562e8, 0x0, 0x1, 0xffffffff7e5b1e37, 0xffffffff7e760fd1,0xffffffff7e73df20), at 0xffffffff7e42fd64
  [6] report_fatal(0xffffffff7e4bf4b9, 0xda4, 0xffffffff7e4bf528, 0xffffffffffc1f758, 0x3e0884, 0x3e0800), at 0xffffffff7e009384
  [7] CMTask::drain_region_stack(0x104a2f8d0, 0x1, 0x0, 0x0, 0xffffffff7dfd8270, 0x1), at 0xffffffff7dfd86a4
  [8] CMTask::do_marking_step(0x1001f8cd0, 0x104a2f8d0, 0x2000, 0xffffffff7e4bfd08, 0xffffffff7e6ee000, 0xffffffff7e7798f0), at 0xffffffff7dfd8ba4
  [9] CMConcurrentMarkingTask::work(0xffffffff705ff570, 0x5, 0x106679000, 0x1001f8cd0, 0xffffffff7e73605c, 0xffffffff7dfda6e8), at 0xffffffff7dfdaa8c
  [10] GangWorker::loop(0x106679000, 0x6, 0xffffffff7e438980, 0x1022a2ff0, 0x1, 0x5), at 0xffffffff7e438a00
  [11] java_start(0x106679000, 0x67a24, 0x37cf, 0xffffffff7e536cb9, 0xffffffff7e6ee000, 0x106163ae0), at 0xffffffff7e30c928

From disassembly, looks like the guarantee was violated because region stack was not empty.

(dbx) x 0xffffffff7dfd86a4-40/20i
0xffffffff7dfd867c: drain_region_stack+0x03ec: ldub [%i0 + 300], %l3 //i0=CMTask* , l3=has_aborted
0xffffffff7dfd8680: drain_region_stack+0x03f0: ldx [%i0 + 24], %o0 //ConcurrentMark*
0xffffffff7dfd8684: drain_region_stack+0x03f4: cmp %l3, 0 // l3=0
0xffffffff7dfd8688: drain_region_stack+0x03f8: bne,pn %icc,drain_region_stack+0x428 ! 0xffffffff7dfd86b8
0xffffffff7dfd868c: drain_region_stack+0x03fc: nop
0xffffffff7dfd8690: drain_region_stack+0x0400: ld [%o0 + 484], %i1
0xffffffff7dfd8694: drain_region_stack+0x0404: cmp %i1, 0 // i1=1
0xffffffff7dfd8698: drain_region_stack+0x0408: be,pn %icc,drain_region_stack+0x428 ! 0xffffffff7dfd86b8
0xffffffff7dfd869c: drain_region_stack+0x040c: mov 3492, %o1
0xffffffff7dfd86a0: drain_region_stack+0x0410: add %l0, -82, %o2
0xffffffff7dfd86a4: drain_region_stack+0x0414: call report_fatal ! 0xffffffff7e009360

Core and logs in /usr/de119005/gctest/drain_stack_failure on v4v-t5220c-sca11.sfbay.

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I don't think the bug that caused 6847956 could also be causing this, so I opened a separate CR.