[CodeGen][BFloat] Add bfloat MVT type

Summary:
This patch adds BFloat MVT support. It also adds fixed and scalable vector MVT
types for BFloat.

This patch is part of a series that adds support for the Bfloat16 extension of the Armv8.6-a architecture, as
detailed here:

https://community.arm.com/developer/ip-products/processors/b/processors-ip-blog/posts/arm-architecture-developments-armv8-6-a

The bfloat type, and its properties are specified in the Arm Architecture
Reference Manual:

https://developer.arm.com/docs/ddi0487/latest/arm-architecture-reference-manual-armv8-for-armv8-a-architecture-profile

Reviewers: aemerson, huntergr, craig.topper, fpetrogalli, sdesmalen, LukeGeeson, ostannard

Reviewed By: ostannard

Subscribers: LukeGeeson, pbarrio, dschuff, kristof.beyls, hiraditya, aheejin, jdoerfert, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D79706