[mlir][NFC] GreedyPatternRewriteDriver: Merge region-based and multi-op-based drivers

Deduplicate large parts of the worklist processing (`GreedyPatternRewriteDriver::processWorklist`).

The new class hierarchy is as follows:
```
          GreedyPatternRewriteDriver (abstract)
                       ^
                       |
      -----------------------------------
      |                                 |
RegionPatternRewriteDriver         MultiOpPatternRewriteDriver
```

Also update the Markdown documentation.

Differential Revision: https://reviews.llvm.org/D141396