[AArch64][SVE] Implement vector tuple intrinsics

Summary:
This patch adds the following intrinsics for creating two-tuple,
three-tuple and four-tuple scalable vectors:

    * llvm.aarch64.sve.tuple.create2
    * llvm.aarch64.sve.tuple.create3
    * llvm.aarch64.sve.tuple.create4

As well as:

    * llvm.aarch64.sve.tuple.get
    * llvm.aarch64.sve.tuple.set

For extracting and inserting scalable vectors from vector tuples. These
intrinsics are intended to be used by the ACLE functions svcreate<n>,
svget and svset.

This patch also includes calling convention support for passing and
returning tuples of scalable vectors to/from functions.

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D75674