Feature request: RegionVisitor and RegionTransformer

[sklam]

Sample implementation used in Numba RVSDG frontend: https://github.com/numba/numba/blob/edfc1b95bf633abb346fbbe265acf2bb20560a6e/numba/core/frontend2/regionpasses.py

RegionVisitor and RegionTransformers enables a hierarchical thinking for compiler pass implementation. This simplifies compiler passes to just 3 abstract methods:

• visit_block(): handle a purely data-dependence block in RVSDG.
• visit_loop(): handle a loop region.
• visit_switch(): handle a switch region.