RFR: 8279668: x86: AVX2 versions of vpxor should be asserted [v4]

[Aleksey Shipilev]

On Mon, 10 Jan 2022 18:25:11 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> Got the SIGILLs on some machines when testing JDK-8279621. That patch started using shorter `vpxor` versions on `UseAVX = 1` path, which tried to use `VEX.256`-encoded `vpxor` instruction that is only available on AVX2. This should be at very least asserted in assembler code. 
>> 
>> Also fixed the implicit `bool` -> `int` conversion to `vector_len`.
>> 
>> Additional testing:
>>  - [x] Linux x86_64 fastdebug `tier1` with `-XX:UseAVX=1`
>
> Aleksey Shipilev has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains five additional commits since the last revision:
> 
>  - Review comments
>  - Merge branch 'master' into JDK-8279668-vpxor-avx2
>  - Add more asserts
>  - Use AVX_256bit literal
>  - Fix

Thank you for reviews!

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