RFR: 8279668: x86: AVX2 versions of vpxor should be asserted [v2]

[Quan Anh Mai]

On Mon, 10 Jan 2022 11:57:10 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 incrementally with one additional commit since the last revision:
> 
>   Use AVX_256bit literal

What I mean is an assert similar to:
https://github.com/openjdk/jdk/blob/e52bdf0a10d05aee93783d8d4e3a3b349cf4c174/src/hotspot/cpu/x86/assembler_x86.cpp#L2032
Thanks.

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