[SelectionDAG] Add MVT::bf16 to getConstantFP()

Summary:
This was probably overlooked in recent bfloat patches.
Needed to handle bf16 constants in SelectionDAG.

  ConstantFP:bf16<APFloat(0)>

Reviewers: stuij

Reviewed By: stuij

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D81779