[Clang][AArch64] Relax SVE bf16 requirement for opaque builtins.

Feature flags protect instructions not datatypes. This means only
builtins associated with +bf16 protected instructions must be guarded.
Those that treat the data as opaque 16-bit values (e.g. loads, store
and shuffles) should be freely available with the underlying SVE
feature.

Author: paulwalker-arm