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

Summary:
Bfloat16 support added for the following intrinsics:
 - LD1
 - LD1RQ
 - LDNT1
 - LDNF1
 - LDFF1

Reviewers: sdesmalen, c-rhodes, efriedma, stuij, fpetrogalli, 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/D82298