RFR: 8266073: Regression ~2% in Derby after 8261804

[Leo Korinth]

My change in 8261804 made processing_is_mt() to always be false when _active_mt_degree is one. This is better because we then need not to execute on another thread. However, at a few places we do not change the _active_mt_degree if processing_is_mt() is false. We could then be stuck at 1 thread. I now change the code to always change the mt degree. 

Derby does no longer regress for g1, and tier 1 passes. I am running tier 1-3 at the moment.

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Commit messages:
 - 8266073: Regression ~2% in Derby after 8261804

Changes: https://git.openjdk.java.net/jdk/pull/3972/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3972&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8266073
  Stats: 10 lines in 3 files changed: 1 ins; 6 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3972.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3972/head:pull/3972

PR: https://git.openjdk.java.net/jdk/pull/3972