Fuse rescales into Add and Conv2d operation

Remove the upscale to int32 before and after the the add operation.
Re-enable fusing of conv2d and rescale that was removed earlier.

Signed-off-by: Per Åstrand <per.astrand@arm.com>
Change-Id: I5e7d9bd99bb3925588b507824d8eb3e6642cc7f0