[AArch64][SVE] Implement punpk[hi|lo] intrinsics

Summary:
Adds the following two intrinsics:
  - int_aarch64_sve_punpkhi
  - int_aarch64_sve_punpklo

This patch also contains a fix which allows LLVMHalfElementsVectorType
to forward reference overloadable arguments.

Reviewers: sdesmalen, rovka, rengolin

Reviewed By: sdesmalen

Subscribers: tschuett, kristof.beyls, hiraditya, rkruppe, psnobl, greened, cfe-commits, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67830

llvm-svn: 373232