[ARM] Basic bfloat support

This patch adds basic support for BFloat in the Arm backend.
For now the code generation relies on fullfp16 being present.

Briefly:
* adds the bfloat scalar and vector types in the necessary register classes,
* adjusts the calling convention to cope with bfloat argument passing and return,
* adds codegen patterns for moves, loads and stores.

It's tested mostly by the intrinsic patches that depend on it (load/store, convert/copy).

The following people contributed to this patch:

 * Alexandros Lamprineas
 * Ties Stuij

Differential Revision: https://reviews.llvm.org/D81373