RFR: 8266561: Remove Compile::_save_argument_registers

[Vladimir Kozlov]

On Wed, 5 May 2021 14:37:18 GMT, Claes Redestad <redestad at openjdk.org> wrote:

> Compile::_save_argument_registers is always false, so I suggest removing it.
> 
> It was used in the past for certain stubs, but the last use appears to have been removed with [JDK-8136406](https://bugs.openjdk.java.net/browse/JDK-8136406)

These are C2 changes and they look fine.

I thought we may do more clean up but C1 may pass must_gc_arguments to `RuntimeStub::new_runtime_stub()`.

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Marked as reviewed by kvn (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/3884