[SelectionDAG] Implement soft FP legalisation for bf16 FP_EXTEND and BF16_TO_FP

As discussed in D151436, it's safe to do this as a simple shift (as is
done in LegalizeDAG.cpp) rather than needing a libcall. The added test
cases for RISC-V previously just triggered an assertion.

Codegen for bfloat_to_double will be slightly improved by D151434.

Differential Revision: https://reviews.llvm.org/D151563