[mlir][IR] Trigger notifyOperationModified for replacements

Each user of the original value is modified in-place. Therefore, the corresponding notification should be triggered.

Also fixes a bug where `RewriterBase::mergeBlocks` did not notify the GreedyPatternRewriteDriver when replacing uses of block arguments. This function does not trigger "operation replaced" notifications, so the GreedyPatternRewriteDriver was not made aware of such IR changes.

Differential Revision: https://reviews.llvm.org/D144549