Add a debug-only verification pass that asserts that C2's Ideal graph is "well-formed" by checking basic, local invariants (e.g. control nodes (CFG nodes) can have multiple data node users but only one control node user, phi nodes must have an input region, If nodes must have two projections, etc.). This pass could be run at different steps of C2's compilation chain, e.g. after each IGVN pass.

Other example candidate invariants can be found at https://github.com/openjdk/jdk/blob/master/src/utils/IdealGraphVisualizer/ServerCompiler/src/main/java/com/sun/hotspot/igv/servercompiler/ServerCompilerScheduler.java (IGV checks some of these invariants while approximating the CFG).