[mlir][Vector] Update the lowering of `vector.transfer_write` to SCF

This change updates the lowering of `vector.transfer_write` to SCF when
scalable vectors are used. Specifically, when lowering
`vector.transfer_write` to a loop of `vector.extractelement` ops, make
sure that the upper bound of the generated loop is scaled by
`vector.vscale`:
```
    %10 = vector.vscale
    %11 = arith.muli %10, %c16 : index
    scf.for %arg2 = %c0 to %11 step %c1
```

For reference, this is the current version (i.e. before this change):
```
    scf.for %arg2 = %c0 to %c16 step %c1
```
Note that this only valid for fixed-width vectors.

Differential Revision: https://reviews.llvm.org/D154226