IVGCVSW-3223 Fix ref convolution performance regression
* Do not use DataLayoutIndexed::GetIndex for weights and inputs,
as it causes a large regression in performance
* It turned out that the calculation of the indexes for the weights
and inputs was the way it was because of an optimization done
many months ago
* Reverted the relevant hunks and added some comments so
we won't make the same mistake again in the future
* Made the GetIndex function inline to speed up other usages
Change-Id: I343b2ef0446993086f58b9dea1f0de0ba2d92216
Signed-off-by: Matteo Martincigh <matteo.martincigh@arm.com>