RFR: 8309306: G1: Move is_obj_dead from HeapRegion to G1CollectedHeap

[Albert Mingkun Yang]

Simple refactoring around `HeapRegion::is_obj_dead`.

Test: hotspot_gc

-------------

Commit messages:
 - g1-is-obj-dead

Changes: https://git.openjdk.org/jdk/pull/14265/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14265&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8309306
  Stats: 37 lines in 4 files changed: 7 ins; 18 del; 12 mod
  Patch: https://git.openjdk.org/jdk/pull/14265.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14265/head:pull/14265

PR: https://git.openjdk.org/jdk/pull/14265