[AArch64][SVE] Implement masked store intrinsics

Summary:
Adds support for codegen of masked stores, with non-truncating
and truncating variants.

Reviewers: huntergr, greened, dmgreen, rovka, sdesmalen

Reviewed By: dmgreen, sdesmalen

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D69378