RFR: 8302462: [REDO] 8297487: G1 Remark: no need to keep alive oop constants of nmethods on stack [v2]

[Richard Reingruber]

> This is a REDO of JDK-8297487 after the issues that cause the BACKOUT were resolved with [JDK-8300915](https://bugs.openjdk.org/browse/JDK-8300915)
> 
> Testing:
> * The reproducer `test/langtools:tier1` was executed without crash for 12h.
> * Multiple iterations of jtreg tests tiers 1-4  on standard platforms and also on ppc64le.

Richard Reingruber has updated the pull request incrementally with one additional commit since the last revision:

  Improve comment

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Changes:
  - all: https://git.openjdk.org/jdk/pull/12561/files
  - new: https://git.openjdk.org/jdk/pull/12561/files/e423ed1a..bad47038

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12561&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12561&range=00-01

  Stats: 7 lines in 2 files changed: 2 ins; 3 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/12561.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12561/head:pull/12561

PR: https://git.openjdk.org/jdk/pull/12561