[AArch64][SVE2] Add the SVE2.1 BF16 instructions

This patch adds the new FEAT_B16B16 feature as well as the
assembly/disassembly for all of the B16B16 instructions:

bfadd:   BFloat16 floating-point add vectors
bfsub:   BFloat16 floating-point subtract vectors
bfmul:   BFloat16 floating-point multiply vectors
bfclamp: BFloat16 floating-point clamp to minimum/maximum number
bfmax:   BFloat16 floating-point maximum
bfmaxnm: BFloat16 floating-point maximum number
bfmin:   BFloat16 floating-point minimum
bfminnm: BFloat16 floating-point minimum number
bfmla:   BFloat16 floating-point fused multiply-add vectors
bfmls:   BFloat16 floating-point fused multiply-subtract vectors

The reference can be found here:
https://developer.arm.com/documentation/ddi0602/2022-09

Differential Revision: https://reviews.llvm.org/D137321