RFR: Filter out invalid pointers picked up by old gen SATB [v2]

[William Kemper]

> In some cases, the SATB barrier used to support old generation marking may capture a reference to an object that is later evacuated. Old gen marking will crash when it tries to mark through the reference.

William Kemper has updated the pull request incrementally with two additional commits since the last revision:

 - Purge SATB in final update refs, rather than SATB filter
 - Revert "Filter out invalid pointers picked up by old gen SATB"
   
   This reverts commit e1fa77a65aa72d7cd22732bb95b73af950b589b1.

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Changes:
  - all: https://git.openjdk.java.net/shenandoah/pull/140/files
  - new: https://git.openjdk.java.net/shenandoah/pull/140/files/e1fa77a6..9cf5b711

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=140&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=140&range=00-01

  Stats: 34 lines in 2 files changed: 18 ins; 15 del; 1 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/140.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/140/head:pull/140

PR: https://git.openjdk.java.net/shenandoah/pull/140