We get correct result by simplifying the code;)

No need to have the outer "if (gid0 < X)" test around the calculation
code generated by Reduction::GenSlidingWindowFunction(). The lhs and
rhs check the gid0 range themselves and that leads to the desired
result for subtraction.

While fixing this I noticed that the handling of empty cells in
division is also wrong. Will fix in another commit.

Change-Id: Ia45bd81e692a17b0453cc79cd4673a00e119562a
Signed-off-by: Michael Meeks <michael.meeks@collabora.com>