LinalgDependenceGraph: add const modifiers to accessors

MLIR const-correctness policy is to avoid having `const` on IR objects.
LinalgDependenceGraph is not an IR object but an auxiliary data structure.
Furthermore, it is not updated once constructed unlike IR objects. Add const
qualifiers to get* and find* methods of LinalgDependenceGraph since they are
not modifying the graph. This allows transformation functions that require the
dependence graph to take it by const-reference, clearly indicating that they
are not modifying it (and that the graph may have to be recomputed after the
transformation).

PiperOrigin-RevId: 277731608