RFR: 8371854: Shenandoah: Simplify WALK_FORWARD_IN_BLOCK_START use [v2]

[Nityanand Rai]

> Replace define/undefine pattern with #ifdef ASSERT block

Nityanand Rai has updated the pull request incrementally with one additional commit since the last revision:

  incorporate feedback

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Changes:
  - all: https://git.openjdk.org/jdk/pull/28333/files
  - new: https://git.openjdk.org/jdk/pull/28333/files/bfdf9e41..27ccdb3c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=28333&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=28333&range=00-01

  Stats: 5 lines in 1 file changed: 2 ins; 2 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/28333.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28333/head:pull/28333

PR: https://git.openjdk.org/jdk/pull/28333