RFR: 8280817: Clean up and unify empty VM operations

[Stefan Karlsson]

On Fri, 28 Jan 2022 09:15:23 GMT, Stefan Karlsson <stefank at openjdk.org> wrote:

> There are a number of VM operations that do nothing, except triggering a safepoint. I'd like to clean up and unify them a bit to:
> 1) Use one parent class which implements the empty doit function and turns off thread oop processing.
> 2) Remove unused VM_Operations
> 3) Don't reuse the VM_None type - there's bug/inconsistency here in that some subsystems report the name as None, while others report the name passed to the constructor
> 4) Remove unused enum values

Thanks for reviewing!

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PR: https://git.openjdk.java.net/jdk/pull/7261