[vector] RFC: Add scalable shapes for Arm Scalable Vector Extension

[Vladimir Ivanov]

Thanks for the clarifications, Alejandro.

> The scalable shape is just a convenient way to avoid having to code the 16 vector lengths that SVE supports. We should always return a concrete VL shape if the maximum VL happens to be 128, 256 or 512 (which honestly, should be 99% of the time).

Sounds good.

> If this is going to be a contentious issue, I think we might as well bite the bullet and write the 16 versions, which are going to be almost identical. They could even be some thin layers over the current scalable vector class.

Agreed, but I'd like to avoid that :-) So far, I don't see a compelling 
reason to introduce them.

Best regards,
Vladimir Ivanov