[SVE] Remove redundant hasBF16 calls from lowering code.

The are several places where hasBF16 is used to protect code that
has no requirement for the +bf16 feature.  The lowering code uses
stock SVE instructions for things like loads and stores and so is
safe even when +bf16 is not available.

NOTE: Currently the nxvbf16 type is not legal unless the +bf16
feature is available, but that isn't an issue because the affected
code is post type legalisation.

NOTE: This patch mirrors previous work that removed the same
redundant protection from isel patterns where the resulting
selection emitted stock SVE instructions.

Differential Revision: https://reviews.llvm.org/D119328