[AArch64][SVE] ACLE: Add bfloat16 to struct load/stores.

This patch contains:
- Support in LLVM CodeGen for bfloat16 types for ld2/3/4 and st2/3/4.
- New bfloat16 ACLE builtins for svld(2|3|4)[_vnum] and svst(2|3|4)[_vnum]

Reviewers: stuij, efriedma, c-rhodes, fpetrogalli

Reviewed By: fpetrogalli

Tags: #clang, #lldb, #llvm

Differential Revision: https://reviews.llvm.org/D82187