[mlir][vector][nfc] Update vector-to-llvm.mlir (#118112)

* Adds extra comments to group Ops
* Unifies the test function naming, i.e.
  * `@vector_{op_name}_{variant}` -> `@{op_name}_{variant}`
* Unifies input variable names (`%input` -> `%arg0`)
* Capitalises LIT variable names (e.g. `%[[insert]]` --> `%[[INSERT]]`)
* Moves `@step_scalable()` _below_ its "fixed-width" counterpart
  (to follow the existing consistency within this file).

There's still some inconsistencies within this file - I'm happy to send
more updates if folks find it useful. But I'd definitely recommend
splitting across multiple PRs (otherwise it's hard to review).

Author: banach-space