[mlir][Vector] Update patterns for flattening vector.xfer Ops (2/N) (#73523)

Updates patterns for flattening `vector.transfer_read` by relaxing the
requirement that the "collapsed" indices are all zero. This enables
collapsing cases like this one:

```mlir
  %2 = vector.transfer_read %arg4[%c0, %arg0, %arg1, %c0] ... :
    memref<1x43x4x6xi32>, vector<1x2x6xi32>
```

Previously only the following case would be consider for collapsing
(all indices are 0):

```mlir
  %2 = vector.transfer_read %arg4[%c0, %c0, %c0, %c0] ... :
    memref<1x43x4x6xi32>, vector<1x2x6xi32>
```

Also adds some new comments and renames the `firstContiguousInnerDim`
parameter as `firstDimToCollapse` (the latter better matches the actual
meaning).

Similar updates for `vector.transfer_write` will be implemented in a
follow-up patch.