[RISCV] SPLAT_VECTOR of bf16 should not require Zvfhmin. (#95357)

The custom lowering converts to f32, splats as f32, then narrows the
vector to bf16. None of that requires Zvfhmin.

Add new bf16 test files without Zvfh/Zvfmin in their RUN lines. I will
remove the bf16 tests from other files in a follow up patch.