[AArch64][SVE] Implement element count intrinsics

Summary:
Adds intrinsics for the following:

    * cntb
    * cnth
    * cntw
    * cntd
    * cntp

Reviewers: sdesmalen, huntergr, dancgr, rengolin, efriedma, rovka

Reviewed By: efriedma

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70967