RFR: Enable remembered set verification during global collections

[William Kemper]

There are two sets of changes here:
 * Fixes to the verifier itself to enable remembered set verification during global collection
 * Fixes to prevent the remembered set scan from parsing unmarked objects after a global collection

These changes pass the dacapo suite with `-XX:+ShenandoahVerify` and pass the shenandoah tier1 tests.

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Commit messages:
 - Merge branch 'earthling-master' into verify-rset-during-global-gc
 - Merge branch 'shenandoah' into verify-rset-during-global-gc
 - Coalesce and fill old objects after final mark in a global collect
 - Only verify first object on next card when last object spans cards
 - Fix infinite loop scenario and invalid limit in verifier
 - Use safe offset when card runs past region top
 - Also use mark context when verifying rset at init mark for global collects
 - Do not read past top of region when looking for last object in card
 - Restore temporary register
 - Preserve and restore original element count for card table barrier
 - ... and 3 more: https://git.openjdk.java.net/shenandoah/compare/f01e568c...d75a315d

Changes: https://git.openjdk.java.net/shenandoah/pull/64/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=64&range=00
  Stats: 132 lines in 8 files changed: 65 ins; 32 del; 35 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/64.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/64/head:pull/64

PR: https://git.openjdk.java.net/shenandoah/pull/64