[MVT][SVE] Scalable vector MVTs (1/3)

This patch adds a few helper functions to obtain new vector
value types based on existing ones without needing to care
about whether they are scalable or not.

I've confined their use to a few common locations right now,
and targets that don't have scalable vectors should never
need to care about these.

Patch by Graham Hunter.

Differential Revision: https://reviews.llvm.org/D32017

llvm-svn: 300838