[mlir] GreedyPatternRewriteDriver: Entry point takes single region

The rewrite driver is typically applied to a single region or all regions of the same op. There is no longer an overload to apply the rewrite driver to a list of regions.

This simplifies the rewrite driver implementation because the scope is now a single region as opposed to a list of regions.

Note: This change is not NFC because `config.maxIterations` and `config.maxNumRewrites` is now counted for each region separately. Furthermore, worklist filtering (`scope`) is now applied to each region separately.

Differential Revision: https://reviews.llvm.org/D142611