[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