[AArch64][SVE] Predicate bfloat16 load patterns with HasBF16

Reviewers: sdesmalen, c-rhodes, efriedma, fpetrogalli

Reviewed By: fpetrogalli

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D82464