[mlir][vector] Add more tests for ConvertVectorToLLVM (3/n) (#102854)
Adds tests with scalable vectors for the Vector-To-LLVM conversion pass.
Covers the following Ops:
* vector.extractelement
* vector.extract
I have also renamed some function names from `@extract_element{}` to
`@extractelement{}` - that's to make a clearer distinction between
tests for `vector.extractelement` (tested by `@extractelement{}`) and
`vector.extract` (tested by `@extract_element{}`).