RFR: 8313224: Avoid calling JavaThread::current() in MemAllocator::Allocation constructor [v2]

[Ioi Lam]

> Summary: Avoid calling `JavaThread::current()` since we already have a `Thread*` in `MemAllocator::_thread`.
> 
> Although the latter is declared to be a `Thead*` and not a `JavaThread*`, the calling context requires the current thread to be a `JavaThread`, and  `MemAllocator::_thread` was initialized to be  `Thread::current`, so I changed the code from 
> 
> 
> _thread(JavaThread::current()),
> 
> 
> to
> 
> 
> assert(Thread::current()->is_Java_thread(), "must be used by JavaThreads only");
> assert(Thread::current() == allocator._thread, "do not pass MemAllocator across threads");
> _thread = JavaThread::cast(allocator._thread);
> 
> 
> I also clean up a few other lines to limit the explicit use of `JavaThread*`.
> 
> This fix improved `java --version` by about 0.6% for my prototype of [JDK-8310823](https://bugs.openjdk.org/browse/JDK-8310823), which allocates ~24000 heap objects at VM start-up.

Ioi Lam has updated the pull request incrementally with one additional commit since the last revision:

  @tschatzl comments

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/15058/files
  - new: https://git.openjdk.org/jdk/pull/15058/files/da211ff9..19d64c57

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=15058&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=15058&range=00-01

  Stats: 3 lines in 1 file changed: 1 ins; 2 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/15058.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15058/head:pull/15058

PR: https://git.openjdk.org/jdk/pull/15058