RFR: 8353174: Clean up thread register handling after 32-bit x86 removal

[Vladimir Ivanov]

On Mon, 31 Mar 2025 10:19:57 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> Various `MacroAssembler` methods have this code to support passing the thread register, and getting it if `noreg` is passed:
> 
> 
>   // determine java_thread register
>   if (!java_thread->is_valid()) {
> #ifdef _LP64
>     java_thread = r15_thread;
> #else
>     java_thread = rdi;
>     get_thread(java_thread);
> #endif // LP64
>   }
> 
> 
> This never happens after 32-bit x86 removal. x86_64 always uses r15_thread. We can clean those up.
> 
> These are also the only major users of `MacroAssembler::get_thread` that we want to remove/rename to avoid falling into traps like [JDK-8353176](https://bugs.openjdk.org/browse/JDK-8353176).
> 
> 
> Additional testing:
>  - [x] Linux x86_64 server fastdebug, `all`

Looks good.

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

Marked as reviewed by vlivanov (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/24323#pullrequestreview-2751551035