[mlir][vector] Refactor vector-transfer-flatten.mlir (nfc) (1/n) (#95743)

The main goal of this and subsequent PRs is to unify and categorize
tests in:
  * vector-transfer-flatten.mlir
  
This should make it easier to identify the edge cases being tested (and
how they differ), remove duplicates and to add tests for scalable
vectors.

The main contributions of this PR:
  * split tests that covered `xfer_read` + `xfer_write` into separate
tests (majority of the existing tests check _one_ xfer Op at a time),
  * organise tests for `xfer_read` and `xfer_write` into separate
    groups (separate with a big bold comment).

Note, all tests (i.e. test cases) are preserved and some new tests are
added. Deletions that you will see in `git diff` correspond to
`xfer_write` and `xfer_read` Ops being extracted to separate functions
(so that there's one xfer Op per function). In particular, the number of
test functions has grown from 26 to 30.

In addition, this PR unifies the tests so that:
  * input variable names are consistent (e.g. make sure that the input
    memref is always `arg`)
  * CHECK lines use similar indentations
  * 2 x tabs are always used for function arguments, 1 x tab for
    function body

Finally, changes in "VectorTransferOpTransforms.cpp" are merely meant to
unify comments and logic between
  * `FlattenContiguousRowMajorTransferWritePattern` and
  * `FlattenContiguousRowMajorTransferReadPattern`.

0 files changed, 0 insertions, 0 deletions