Adds a new experimental vscale intrinsic to represent the runtime
multiple for scalable vectors.
Implements lowering/legalization for this in the AArch64 backend,
and a unit test for the SVE 'rdvl' instruction over scalable integer
Part of the initial SVE codegen series, rfc will be posted soon. Note: this patch is currently just to support the scalable vector codegen rfc, and does not need a full review yet.