[RISCV] Guard LowerINSERT_VECTOR_ELT against fixed vectors.

The type legalizer can call this code based on the scalar type so
we need to verify the vector type is a scalable vector.

I think due to how type legalization visits nodes, the vector type
will have already been legalized so we don't have an issue with
using MVT here like we did for EXTRACT_VECTOR_ELT.
I've added a test just in case.