[mlir][Linalg] NFC: Refactor LinalgDependenceGraphElem to allow

representing dependence from producer result to consumer.

With Linalg on tensors the dependence between operations can be from
the result of the producer to the consumer. This change just does a
NFC refactoring of the LinalgDependenceGraphElem to allow representing
both OpResult and OpOperand*.

Differential Revision: https://reviews.llvm.org/D95208