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.