[mlir][vector] Add more tests for ConvertVectorToLLVM (10/n) (#117041)

Adds tests with scalable vectors for the Vector-To-LLVM conversion pass.
Covers the following Ops:

  * `vector.maskedload`,
  * `vector.maskedstore`,
  * `vector.gather`,
  * `vector.scatter`.

In addition:
* For consistency with other tests, renamed test function names
  (e.g. `@masked_load_op` -> `@masked_load_op`)
* Made some test names more descriptive, e.g `@gather_op_2d` ->
  `@gather_1d_from_2d`.