MLBEDSW-9183: Improve block-config selection for Matmul

- Remove Small 4x4 IFM constraint to allow larger blocks
  for Matmul operations.
- Consider OFM-coverage instead of IFM-fetch in relativeCost
  calculations.

Change-Id: I5aee67f35bd979cc77328afb22b254dbdd0cabdd
Signed-off-by: Alexander Bengtsson <Alexander.Bengtsson@arm.com>