[vector] RFR: Remove VShiftV nodes

[John Rose]

It’s like watching the gravitational collapse of some
celestial body, as the code folds smaller and smaller.
Hopefully it stays compact.

> On Apr 9, 2020, at 1:19 PM, Vladimir Ivanov <vladimir.x.ivanov at oracle.com> wrote:
> 
> http://cr.openjdk.java.net/~vlivanov/panama/vector/vshift/webrev.00/
> 
> Get rid of dedicated nodes for variable shifts and reuse existing [UR|R|L]ShiftV nodes.
> 
> Overall, it simplifies the code except detecting when variable shifts are supported. Introduced Matcher::supports_vector_variable_shifts() for that.
> 
> The patch is against vectorIntrinsics branch.
> 
> Best regards,
> Vladimir Ivanov