[RISCV] Fix insert_subvector with fixed vector type creating invalid node (#82975)

If the vector type is a fixed vector type, we convert it to a container
scalable vector type to compute its reg class. But we need to keep the
old
fixed type so we create a result node with the same type.

This code path is currently dead so I haven't been able to create a test
case
for it. But I have an upcoming patch for insert_subvector lowering that
will
exercise this.