Add dominators, SSA and def-use chains #15

SSA construction is a transformation over the non-SSA CFG.

• Dominators, immediate dominators, dominance frontiers
• Post-dominators and control dependence (post-dominance frontier)
• Phi insertion on the iterated dominance frontier, pruned by liveness, with renaming of local values; undefined for paths without a definition
• Def-use and use-def chains over SSA values
• cfg.dominance, cfg.post_dominance, ir.ssa and ir.defuse declared through the Analysis Manager with explicit requires

Reference: docs/architecture.md §5 CFG, §7 SSA.

Delivery: #16