[jdk22u] RFR: 8329570: G1: Excessive is_obj_dead_cond calls in verification

[Aleksey Shipilev]

Simple fastdebug performance improvement fix. Risk is low, as it only moves the assert to an earlier stage.

Additional testing:
 - [ ] GHA

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Commit messages:
 - Backport 29992e1fca4faef0fe0ac453c7e78db4628d5e70

Changes: https://git.openjdk.org/jdk22u/pull/132/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk22u&pr=132&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8329570
  Stats: 2 lines in 1 file changed: 1 ins; 1 del; 0 mod
  Patch: https://git.openjdk.org/jdk22u/pull/132.diff
  Fetch: git fetch https://git.openjdk.org/jdk22u.git pull/132/head:pull/132

PR: https://git.openjdk.org/jdk22u/pull/132