[ARM][BFloat] Lowering of create/get/set/dup intrinsics

This patch adds codegen for the following BFloat
operations to the ARM backend:
* concatenation of bf16 vectors
* bf16 vector element extraction
* bf16 vector element insertion
* duplication of a bf16 value into each lane of a vector
* duplication of a bf16 vector lane into each lane

Differential Revision: https://reviews.llvm.org/D81411