RFR: 8279008: G1: Calculate BOT threshold on-the-fly during Object Copy phase

[Albert Mingkun Yang]

Simple change of unifying BOT logic for PLAB and direct allocation during GC.

Test: tier1-6; no difference observed while running BigRamTester, SPECjbb2015

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Commit messages:
 - plab-bot

Changes: https://git.openjdk.java.net/jdk/pull/6916/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6916&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8279008
  Stats: 131 lines in 11 files changed: 11 ins; 102 del; 18 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6916.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6916/head:pull/6916

PR: https://git.openjdk.java.net/jdk/pull/6916