[RISCV] Add FP cost model tests for no zfhmin/zfbfmin. NFC

Vector costs without zvfhmin/zvfbfmin and zfhmin/zfbfmin are somehow
cheaper than with zvfhmin/zvfbfmin at smaller vector sizes, despite the
fact that the former are scalarized to libcalls. This adds a RUN line to
showcase this, splitting out the bfloat tests into their own functions
so we don't have duplicate lines for the regular float/double costs.

Author: lukel97