[mlir][scf] Relax requirements for loops fusion (#79187)

Enable the fusion of parallel loops also when the 1st loop contains
multiple write accesses to the same buffer, if the accesses are always
on the same indices.
Fix LIT test cases whose loops were not being fused.

Signed-off-by: Fabrizio Indirli <Fabrizio.Indirli@arm.com>