RFR: 8241502: Migrate x86_64.ad to MacroAssembler

[Dean Long]

On Thu, 11 Feb 2021 05:12:55 GMT, John Tortugo <github.com+2249648+JohnTortugo at openjdk.org> wrote:

>> I wish there was a way for the old and new versions to co-exist at the same time, so we could generate the code the old way and and the new way, then compare, for automatic verification of the MacroAssember version.
>
> Thank you all for the feedback!
> 
> @iklam - I'll check that and let you know once I make more conversions.
> 
> @dean-long - That would be great. I'm all ears for the best way to test this!

Here's one way to test both versions, using loadRange as an example.  It's not exactly pretty, but it seems to work.
[loadRange.txt](https://github.com/openjdk/jdk/files/5994725/loadRange.txt)

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PR: https://git.openjdk.java.net/jdk/pull/2420