[AArch64][SVE] Implement ptrue intrinsic

Reviewers: sdesmalen, eli.friedman, dancgr, mgudim, cameron.mcinally,
huntergr, efriedma

Reviewed By: sdesmalen

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71457