RFR: 8317318: Serial: Change GenCollectedHeap to SerialHeap in whitebox

Fri Sep 29 15:37:13 UTC 2023

Use more precise type for Serial GC. I also added a ` ShouldNotReachHere()` there, because using serial-heap when serial-gc is not used seems problematic.

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Commit messages:
 - s1-prims

Changes: https://git.openjdk.org/jdk/pull/15988/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15988&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8317318
  Stats: 10 lines in 1 file changed: 7 ins; 1 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/15988.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15988/head:pull/15988

PR: https://git.openjdk.org/jdk/pull/15988