RFR: 8317755: G1: Periodic GC interval should test for the last whole heap GC

[Aleksey Shipilev]

See the description in the bug. Fortunately, we already track the last whole-heap GC. The new regression test verifies the behavior.

Additional testing:
 - [ ] Linux x86_64 fastdebug `tier1 tier2 tier3`

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

Commit messages:
 - Keep the cast
 - Fix

Changes: https://git.openjdk.org/jdk/pull/16107/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16107&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8317755
  Stats: 161 lines in 2 files changed: 158 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/16107.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16107/head:pull/16107

PR: https://git.openjdk.org/jdk/pull/16107