Add encoder/decoder support for Arm's Scalable Vector Extension

Created by: fhahn

Armv8-a's SVE is a big extension requiring a few changes to the AArch64 backend. At first, I think we need new register types for scalable vector (Z) and predicate (P) registers as well as support for the new SVE encodings.

https://developer.arm.com/docs/ddi0584/latest/arm-architecture-reference-manual-supplement-the-scalable-vector-extension-sve-for-armv8-a

Xref #2626