[MLIR][Affine] Generalize/fix mdg init for region-holding ops with known control flow (#71754)

Generalize/fix mdg init for region-holding ops with well-defined control
flow. Use the memory effect interface to determine when to create a
node. While on this, remove the special treatment there for call ops.

This allows fusion of affine nests even in the presence of scf region
ops elsewhere in the Block. Previously, even a single scf.for/if/while
op in a block would have made fusion bail out on all affine fusion that
was possible. Addressed.