[MLIR][Affine] Fix/improve affine sibling fusion

The sibling fusion profitability checks shouldn't rely on the presence
of a store op in the sibling. The reuse is between the loads.

Fixes issues raised at https://discourse.llvm.org/t/understanding-the-affine-loop-fusion-pass/69452

Reviewed By: dcaballe

Differential Revision: https://reviews.llvm.org/D146763