CCP, DCE, GVN, IR Builder API, improve verification, rewrite dot visualization

• Add conditional constant propagation - implemented as 1. a dataflow analysis over reachability + interpreted constant lattices 2. propagate uses of constants
• Add dead code elimination to delete nodes with 0 users - implemented as worklist
• Add GVN with identity function simplification (for a very limited set of identity functions for now)
• Add an API for building IR - mainly does interning
• Add some check to verification, including checking dynamic constants
• Rewrite dot visualization tool - refactor to make code cleaner, generate better graphs