This moves the different canonicalizations of regions into one place and invokes them in the fixed-point iteration of the canonicalizer.

PiperOrigin-RevId: 281617072

This is a simple multi-level DCE pass that operates pretty generically on
the IR. Its key feature compared to the existing peephole dead op folding
that happens during canonicalization is being able to delete recursively
dead cycles of the use-def graph, including block arguments.

PiperOrigin-RevId: 281568202