RFR: 8358735: GenShen: bug in #undef'd code in block_start() [v5]

[Kelvin Nilsen]

> When scanning a range of dirty cards within the GenShen remembered set, we need to find the object that spans the beginning of the left-most dirty card.  The existing code is not reliable following class unloading.
> 
> The new code uses the marking context when it is available to determine the location of live objects that reside below TAMS within each region.  Above TAMS, all objects are presumed live and parsable.

Kelvin Nilsen has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 29 commits:

 - Merge remote-tracking branch 'jdk/master' into finish-block-start
 - fix bugs in implementation of weakly referenced object handling
 - Fixup handling of weakly marked objects in remembered set
 - fix idiosyncratic formatting
 - cleanup code for review
 - Fix order of include files
 - fix white space
 - disable for debug build, alphabetic order for includes
 - add explicit typecast to avoid compiler warning message
 - Remove troublesome assert that assumes lock is held
 - ... and 19 more: https://git.openjdk.org/jdk/compare/13b3d2fc...d341522e

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Changes: https://git.openjdk.org/jdk/pull/27353/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27353&range=04
  Stats: 871 lines in 11 files changed: 831 ins; 6 del; 34 mod
  Patch: https://git.openjdk.org/jdk/pull/27353.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27353/head:pull/27353

PR: https://git.openjdk.org/jdk/pull/27353