[RISCV] Reorder checks in RISCVTTIImpl::getGatherScatterOpCost to avoid...

[RISCV] Reorder checks in RISCVTTIImpl::getGatherScatterOpCost to avoid calling getMinRVVVectorSizeInBits() when V extension is not enabled.

getMinRVVVectorSizeInBits() asserts if the V extension isn't
enabled. So check that gather/scatter is legal first since it
already contains a check for V extension being enabled. It
also already checks getMinRVVVectorSizeInBits for fixed length
vectors so we don't need a check in getGatherScatterOpCost.