RFR: 8258961: devirtualize SafePointNode::jvms() and hide out set_jvms() [v3]

[Xin Liu]

> SafePointNode::jvms() declares virtual but has never been overridden. It seems unnecessary.
> The member variable _jvms declares 'JVMState* const', but set_jvms() updates its value anyway.
> Geting rid of the const qualifier and hides if out using protected.

Xin Liu has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:

 - Merge branch 'master' into JDK-8258961
 - 8258961: devirtualize SafePointNode::jvms() and hide out set_jvms()
   
   devirtualize SafePointNode::jvms(). We can't hide the member variable _jvms
   because it is exposed to HotSpot Servicability agent. keeping the qualifier
   const for the same resaon.
 - 8258961: devirtualize SafePointNode::jvms() and hide out set_jvms()

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/1899/files
  - new: https://git.openjdk.java.net/jdk/pull/1899/files/b42b4689..f393855d

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=1899&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=1899&range=01-02

  Stats: 5467 lines in 499 files changed: 1261 ins; 3170 del; 1036 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1899.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1899/head:pull/1899

PR: https://git.openjdk.java.net/jdk/pull/1899