[RISCV] Support fixed vector VP_LOAD/STORE for bf16 and f16 without Zvfh. (#107297)
This allows odd sized vector load/store to be legalized to a
VP_LOAD/STORE using EVL.
I changed the bf16 tests in fixed-vectors-load.ll and
fixed-vectors-store.ll to use an illegal type to be consistent with the
intent of these files. A legal type is already tested in
fixed-vectors-load-store.ll