MLBEDSW-4350 Use padding instead of skirt for merged SplitSlice

When the operations are merged some later passes are confused by start
and end coordinates for the convolution not being along the edges of
the IFM, and omitting padding. But we need the zero padding to keep
the output the same as before the transformation.
Also fixes bug where Vela could crash if convolution had explicit
start coordinate.

Signed-off-by: Henrik G Olsson <henrik.olsson@arm.com>
Change-Id: I8449d237350d528f83738b2f09124f1ed79c07ca