[AArch64][SME] Fix v8.6a bf16 NEON instruction predication

In streaming mode on SME targets only the scalar BFCVT armv8.6-a
instruction is legal, predicate the illegal instructions on NEON to
disable them in streaming mode (see D107902). BFCVT is predicated on
HasNEONorStreamingSVE.

The reference can be found here:
https://developer.arm.com/documentation/ddi0602/2021-06/SIMD-FP-Instructions

Reviewed By: paulwalker-arm

Differential Revision: https://reviews.llvm.org/D108279