Adds the following:
- experimental_vector_stepvector intrinsic, which represents a runtime vector containing a monotically increasing sequence of the form <0, 1, 2, ....n-1>, where 'n' is the total number of elements of the given type.
- AArch64ISD::SERIES_VECTOR, a target specific ISD to represent this sequence concept in the AArch64 backend, along with code to lower the stepvector intrinsic to this node.
- ISel patterns to select 'index' instructions for series_vector nodes.
- Unit tests for scalable vectors of the various integer types.
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.