RFR: 8256203: Simplify RegMask::Empty

[Claes Redestad]

On Thu, 12 Nov 2020 10:15:00 GMT, Tobias Hartmann <thartmann at openjdk.org> wrote:

>> - Simplify RegMask::Empty to use default constructor.
>> - Add missing validation in the empty constructor.
>
> Looks good to me.

@TobiHartmann @chhagedorn - thank you for reviewing!

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PR: https://git.openjdk.java.net/jdk/pull/1167