[RISCV] Remove RISCVII:VSEW enum. Make encodeVYPE operate directly on SEW.

The VSEW encoding isn't a useful value to pass around. It's better
to use SEW or log2(SEW) directly. The only real ugliness is that
the vsetvli IR intrinsics use the VSEW encoding, but it's easy
enough to decode that when the intrinsic is processed.