[AArch64][SVE] Add bfloat16 support to store intrinsics

Summary:
Bfloat16 support added for the following intrinsics:
 - ST1
 - STNT1

Reviewers: sdesmalen, c-rhodes, fpetrogalli, efriedma, stuij, david-arm

Reviewed By: fpetrogalli

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

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D82448