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

[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`

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Commit messages:
 - Fix

Changes: https://git.openjdk.java.net/jdk/pull/7005/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7005&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8279668
  Stats: 8 lines in 1 file changed: 6 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