[RISCV] Reduce dependency on RISCV::RVVBitsPerBlock for calculating vector size for...

[RISCV] Reduce dependency on RISCV::RVVBitsPerBlock for calculating vector size for -mrvv-vector-bits.

We can use the minimum value of the BuiltinType's ElementCount and
the element size.

This needs to be done to support LMUL!=1 types anyway.

I did have to make an ordering change in the error checks in
HandleRISCVRVVVectorBitsTypeAttr to check if the type is an RVV
VLS type before checking the size.