[RISCV] Remove Zfbfmin dependency from Zvfbfmin. (#75851)

Zvfbfmin does not have any scalar operands making this an unnecessary
dependency. The spec was just updated to remove this. See
https://github.com/riscv/riscv-bfloat16/commit/86d7a74f4b928e981f79f6d84a4592e6e9e4c0e9

This fixes a correctness issue where Xsfvfwmaccqqq was incorrectly
depending on Zfbfmin. The SiFive CPUs that support Xsfvfwmaccqqq do not
implement Zfbfmin, but do implement Zvfbfmin based on a previous
understanding that it only requires Zve32f. I've added tests for this
feature to raise the bar for adding dependencies to it in the future.