Outlining

• Implement outlining routine. Handles all valid cases, including multiple entry / exit points. Currently uses Undefs and accepts / returns all possible data items. Accepts / returns a control indicator to communicate control flow. TODO: use summation types instead.
• Dumby outlining pass to outline all of each function. This is so the original entry function can get lowered by the Rust backend, and the new non-entry function can get lowered using a new CPU backend.

A cool demo: cargo run --bin hercules_driver hercules_samples/matmul/src/matmul.hir "Xdot(false) Outline Xdot(false) Inline DeleteUncalled SROA DCE Xdot(false)".

hercules_ir/src/def_use.rs

@@ -220,6 +220,7 @@ pub fn get_uses(node: &Node) -> NodeUses {
             control,
             selection: _,
         } => NodeUses::One([*control]),
+        Node::Undef { ty: _ } => NodeUses::One([NodeID::new(0)]),
     }
 }
 
@@ -319,5 +320,6 @@ pub fn get_uses_mut<'a>(node: &'a mut Node) -> NodeUsesMut<'a> {
             control,
             selection: _,
         } => NodeUsesMut::One([control]),
+        Node::Undef { ty: _ } => NodeUsesMut::Zero,
     }
 }