[mlir][ArmSME] Fix loop bounds of masked loads/stores (#78983)

Previously, for masked tile loads/stores we directly used the dimension
size from the `vector.create_mask` operation as the upper bound of the
`scf.for` over the tile slices. This was not correct, as `create_mask`
allows operands to be greater than the size of the vector dimension, in
which case the for loop bounds should be clamped to the number of tile
slices.