RFR: 8358735: GenShen: bug in #undef'd code in block_start() [v3]

[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 incrementally with one additional commit since the last revision:

  Fixup handling of weakly marked objects in remembered set

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Changes:
  - all: https://git.openjdk.org/jdk/pull/27353/files
  - new: https://git.openjdk.org/jdk/pull/27353/files/668e8615..80198abe

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=27353&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=27353&range=01-02

  Stats: 32 lines in 1 file changed: 20 ins; 1 del; 11 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