Implements the following intrinsics:
- int_aarch64_sve_sunpkhi
- int_aarch64_sve_sunpklo
- int_aarch64_sve_uunpkhi
- int_aarch64_sve_uunpklo
This patch also adds AArch64ISD nodes for UNPK instead of implementing
the intrinsics directly, as they are required for a future patch which
implements the sign/zero extension of legal vectors.
This patch includes tests for the Subdivide2Argument type added by D67549
Where is op used? I assume that comes later but it would help to understand where this is going.