RFR: 8371854: Shenandoah - Simplify WALK_FORWARD_IN_BLOCK_START use

[Nityanand Rai]

Replace define/undefine pattern with #ifdef ASSERT block

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Commit messages:
 - Update JDK-8371854 fix: Simplify WALK_FORWARD_IN_BLOCK_START macro usage

Changes: https://git.openjdk.org/jdk/pull/28333/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28333&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8371854
  Stats: 6 lines in 1 file changed: 0 ins; 4 del; 2 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