[mlir][vector][test] Update tests for vector.xfter_{read|write} (#91943)

Updates tests in "vector-transfer-permutation-lowering.mlir" to make a
clearer split into cases for :

* xfer_read vs xfer_write
* fixed-width vs scalable tests

A new test case is added for fixed-width vectors for vector.transfer_read.
This is to complement an existing test for scalable vectors.

This is in preparation for #90835 and also for adding more tests for
scalable vectors.