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

[Aleksey Shipilev]

> 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

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/7005/files
  - new: https://git.openjdk.java.net/jdk/pull/7005/files/3012705b..e52bdf0a

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=7005&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=7005&range=00-01

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7005.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7005/head:pull/7005

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