Fix dominator based verification, add structural checks

IR.md

hercules_ir/src/def_use.rs

hercules_ir/src/dot.rs

hercules_ir/src/subgraph.rs

hercules_ir/src/typecheck.rs

hercules_ir/src/verify.rs

hercules_tools/src/hercules_dot/main.rs

samples/invalid/bad_phi.hir

samples/simple1.hir