[AArch64][SVE] Implement int_aarch64_sve_cnt intrinsic

Summary: This patch includes tests for the VecOfBitcastsToInt type added by D68021

Reviewers: c-rhodes, sdesmalen, rovka

Reviewed By: c-rhodes

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68023

llvm-svn: 373468