RFR (XXS) 8234610: MaxVectorSize set wrongly when UseAVX=3 is specified after JDK-8221092

[Viswanathan, Sandhya]

On Skylake platform the JVM by default sets the UseAVX level to 2 and accordingly sets MaxVectorSize to 32 bytes as per JDK-8221092<https://bugs.openjdk.java.net/browse/JDK-8221092>.

When the user explicitly wants to use AVX3 on Skylake platform, they need to invoke the JVM with -XX:UseAVX=3 command line argument.
This should automatically result in MaxVectorSize being set to 64 bytes.

However post JDK-8221092<https://bugs.openjdk.java.net/browse/JDK-8221092>, when -XX:UseAVX=3 is given on command line, the MaxVectorSize is being wrongly set to 16 bytes.
I have a patch which fixes the issue.

JBS: https://bugs.openjdk.java.net/browse/JDK-8234610
Webrev: http://cr.openjdk.java.net/~sviswanathan/8234610/webrev.00/

Please review and approve.

Best Regards,
Sandhya