[mlir][vector] Update tests for collapse 6/n (nfc) (#98902)

The main goal of this PR (and subsequent PRs), is to add more tests with
scalable vectors to:
  * vector-transfer-collapse-inner-most-dims.mlir

There's quite a few cases to consider, hence this is split into multiple
PRs.

In this PR, I am making the following changes:
* All input memrefs for `xfer_read` are are renamed as `%src`.
* All input memrefs for `xfer_write` are are renamed as `%dest`.
* All variables representing pad values for `xfer_read` are renamed as
  `%pad`.
* All vector variables (for `xfer_read` and `xfer_write`) are renamed as
  `%v`.
* Add `@contiguous_inner_most_non_zero_idx_in_bounds_scalable` for
  `xfer_read` (similar test already exists for `xfer_write`)
* All index variables are renamed as `%i` (1st index) and `%ii` (2nd
  index).

The above were marked as TODOs in the test file - these are not
resolved. In addition (to avoid sending another PR):
* `@drop_inner_most_dim` is deleted - it duplicates
  `@contiguous_inner_most` for xfer_write
* For consistency with other negative tests, renamed `@non_unit_strides`
  as `@negative_non_unit_strides` and added a similar test for
  `xfer_read`
* `@non_unit_strides` is renamed as `@negative_non_unit_strides` and
  a similar test is added for `xfer_read`.

This is a follow-up for: #94490, #94604, #94906, #96214, #96227

Author: banach-space