C2 SuperWord: introduce VerifyAlignVector (runtime alignment check)

I have recently found a bug with our AlignVector JDK-8310190.

Machines that actually require AlignVector are rare. Hence, I want to add a runtime verification that checks alignment on other machines / platforms. This will give me the confidence to fix the bug. Because it will require a substantial refactoring.

That refactoring will be generally beneficial, as it will untangle the pair/pack generation from alignment concerns in general.

Related issues: JDK-8303827 JDK-8309662