RFR: 8252406: Introduce Thread::as_Java_thread() convenience function [v4]

[David Holmes]

> This is a rather large but generally simple cleanup.
> 
> We use a lot of raw C-style casts to "(JavaThread*)" typically after checking "thread->is_Java_thread()". To simplify
> this pattern, and make the code look somewhat cleaner we introduce a convenience function Thread::as_Java_thread(),
> which asserts is_Java_thread() and returns "this" cast to "JavaThread*". A const version, as_const_Java_thread(), was
> also added to allow use in cases where we start with "const Thread *".  Summary of changes:
> - changed raw casts to use as_Java_thread()
> - removed redundant checks of is_Java_thread() as it is now done in as_Java_thread()
> - Removed checks that are checking the type system e.g.
> void foo(JavaThread* t) {
>   assert(t->is_Java_thread(), "must be")
> the only way the assert can fire is if someone deliberately bypasses the type system, and if we are going to worry
> about that then we would need asserts like this on every method that expects a JavaThread! The right place for the
> assertion is at the head of any such call chain.
> - Removed asserts that are already guaranteed in the caller.
> - Use JavaThread::current() where appropriate.
> - Use pre-existing "thread" variable where available rather than casting THREAD
> 
> Change locations found by grepping for variations of the cast syntax "(JavaThread*)" - it is possible some may have
> been missed.
> Testing: tiers 1-3
> 
> Thanks,
> David

David Holmes has updated the pull request incrementally with one additional commit since the last revision:

  This reverts commit 689951708fccd0073873e645bf683bc34b75a772
  This reverts commit da70f8047d3f9c8af9d3eee3b14c71122c5220a0.
  
  Reverting v3 and v2 so that we can take a simpler approach that touches far fewer files.

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/37/files
  - new: https://git.openjdk.java.net/jdk/pull/37/files/68995170..202af760

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=37&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=37&range=02-03

  Stats: 110 lines in 40 files changed: 28 ins; 45 del; 37 mod
  Patch: https://git.openjdk.java.net/jdk/pull/37.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/37/head:pull/37

PR: https://git.openjdk.java.net/jdk/pull/37