Adds tests with scalable vectors for the Vector-To-LLVM conversion pass.
Covers the following Ops:
* `vector.splat`.
In addition:
* Removed `@make_fixed_vector_of_scalable_vect`, which duplicated
`@broadcast_vec2d_from_scalar_scalable` (and wasn't grouped with other
tests for `vector.broadcast`).
* Moved `@vector_bitcast_2d` near other tests for `vector.bitcast` and
added a variant with scalable vectors.