RFR: 8258961: devirtualize SafePointNode::jvms() [v4]

[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()
  
  remove the virtual qualifier from Node::jvms(). check its type is SafePointNode and
  MachSafePointNode instead.
  
  This patch also move some fiels to proctected zone and declare VMStructs is a friend.

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/1899/files
  - new: https://git.openjdk.java.net/jdk/pull/1899/files/f393855d..44d7b8e7

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=1899&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=1899&range=02-03

  Stats: 81 lines in 8 files changed: 34 ins; 22 del; 25 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