RFR: 8279668: x86: AVX2 versions of vpxor should be asserted

[Aleksey Shipilev]

On Mon, 10 Jan 2022 10:58:53 GMT, Quan Anh Mai <duke at openjdk.java.net> wrote:

> Hi, I think the assert should be inserted in the assembler instead. 

I think `Assembler::vpxor` actually does it right already, and the problem is only in "shortcut" macro-method for AVX2. So the assert is where it should be.

> Also, I would prefer `AVX_256bit` instead of `1`. Thanks.

Good idea, fixed.

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PR: https://git.openjdk.java.net/jdk/pull/7005