Clone elimination pass

• Very simple clone elimination pass.
• Rewrite matmul to not use implicit clones. Admitting defeat for now.

hercules_opt/src/clone_elim.rs

+extern crate hercules_ir;
+
+use std::collections::BTreeSet;
+
+use self::hercules_ir::ir::*;
+
+use crate::*;
+
+/*
+ * Top level function to run clone elimination.
+ */
+pub fn clone_elim(editor: &mut FunctionEditor) {
+    // Create workset (starts as all nodes).
+    let mut workset: BTreeSet<NodeID> = (0..editor.func().nodes.len()).map(NodeID::new).collect();
+
+    while let Some(work) = workset.pop_first() {
+        // Look for Write nodes with identical `collect` and `data` inputs.
+        let nodes = &editor.func().nodes;
+        if let Node::Write {
+            collect,
+            data,
+            ref indices,
+        } = nodes[work.idx()]
+            && nodes[collect.idx()] == nodes[data.idx()]
+        {
+            assert!(indices.is_empty());
+            editor.edit(|edit| edit.replace_all_uses(work, collect)?.delete_node(work));
+
+            // Removing this write may affect downstream writes.
+            workset.extend(editor.get_users(work));
+        }
+    }
+}