[VE] VEC_BROADCAST, lowering and isel

This defines the vec_broadcast SDNode along with lowering and isel code.
We also remove unused type mappings for the vector register classes (all vector MVTs that are not used in the ISA go).

We will implement support for short vectors later by intercepting nodes with illegal vector EVTs before LLVM has had a chance to widen them.

Reviewed By: kaz7

Differential Revision: https://reviews.llvm.org/D91646