MLBEDSW-7343: MLCE: Unsupported STRIDED_SLICE with negative index and shrinking an axis

 - The problem was that the end values of STRIDED_SLICE operators
were not taking the shrink_axis_mask into account
 - The fix is simply to ignore the end value set on the operator
and calculate one based upon shrinking the axis

Change-Id: I2e5f2d3c9b08035dfd9b1629c775408f2356d1cf
Signed-off-by: Tim Hall <tim.hall@arm.com>