[RISCV] Adjust fixed vector coverage for get.active.lane.mask

Make sure we include at least one case where the vsadd/vmsltu lowering
requires only LMUL1.  We should be able to generate all of the fixed
vector variants from scalar to vector idioms, but this is probably not
very important right now given the fixed length variants we'd actually
use when vectorizing with LMUL=1 are reasonable.