[RISCV] Add tests for rv32 gather/scatter costs. NFC
There's a divergence with the rv32 costs that I plan on fixing in
another patch, so this precommits the tests for them.
The zve32f RUN lines were split off into another file so the check prefixes
are easier to reason about.
The -riscv-v-vector-bits-max RUN lines were also removed to simplify the
check prefixes since I'm not sure if they were intentionally testing any
specific logic.
