RFR: 8258961: devirtualize SafePointNode::jvms() and hide out set_jvms() [v2]

[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 incrementally with one additional commit since the last revision:

  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.

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/1899/files
  - new: https://git.openjdk.java.net/jdk/pull/1899/files/bdbeda12..b42b4689

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=1899&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=1899&range=00-01

  Stats: 33 lines in 5 files changed: 11 ins; 13 del; 9 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