Reorganization, global code motion, backward dataflow, loop tree construction, dom tree improvements, collapse region chains in CCP

• Reorganized repo into hercules_ir, hercules_opt, and hercules_codegen
• Implemented global code motion, as described in Click's thesis (nodes are placed as loop invariant, and then as control dependent as possible)
• Generalized dataflow core code to work with backward dataflow, created backward dataflow helper
• Construct loop tree from dom tree - identify loop back edges, identify natural loops, and identify tree structure
• Various enhancements to dom tree (common ancestor, lowest amongst, chain iterator)
• Collapse region chains during CCP
• Various improvements to other parts of compiler, such as subgraph