[RISCV] Expand test coverage of bf16 operations with Zfbfmin and fix gaps

This doesn't bring us to parity with the test/CodeGen/RISCV/half-* test
cases, it simply picks off an initial set that can be supported
especially easy. In order to make the review more manageable, I'll
follow up with other cases.

There is zero innovation in the test cases - they simply take the
existing half/float cases and replace f16->bf16 and half->bfloat.

Differential Revision: https://reviews.llvm.org/D156895