[mlir][vector][nfc] Update vector-to-llvm.mlir

* 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`)

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