RFR(S): 8213538: VM crashes when MaxVectorSize is set to 0, 1 or 2

[Nils Eliasson]

Hi,

This patch add the requirement that MaxVectorSize must be at least 4 on 
x64.

The background is that we crash in startup when testing when setting it 
to 0, 1 or 2. When MaxVectorSize is less than 4, the XMM reg types won't 
be initialized, and then we crash in intrinsics that unconditionally use 
XMM regs. Since SSE2 is the required minimum on x64, the assumption that 
you can use XMM regs is always valid. This adds the requirement that 
MaxVectorSize is at least 4.

I had to do a little refactoring to make the code more readable, but the 
essence is the same.

Bug: https://bugs.openjdk.java.net/browse/JDK-8213538

Webrev: http://cr.openjdk.java.net/~neliasso/8213538/webrev/

Testing is running.

Please review,

// Nils