Eliminate call stack recursion in VirtRegs::find (#584)

* Eliminate call stack recursion in VirtRegs::find

lib/codegen/src/regalloc/virtregs.rs

@@ -291,21 +291,23 @@ impl UFEntry {
 impl VirtRegs {
     /// Find the leader value and rank of the set containing `v`.
     /// Compress the path if needed.
-    fn find(&mut self, val: Value) -> (Value, u32) {
+    fn find(&mut self, mut val: Value) -> (Value, u32) {
+        let mut val_stack = vec![];
+        let found = loop {
             match UFEntry::decode(self.union_find[val]) {
-            UFEntry::Rank(rank) => (val, rank),
+                UFEntry::Rank(rank) => break (val, rank),
                 UFEntry::Link(parent) => {
-                // TODO: This recursion would be more efficient as an iteration that pushes
-                // elements onto a SmallVector.
-                let found = self.find(parent);
-                // Compress the path if needed.
-                if found.0 != parent {
+                    val_stack.push(val);
+                    val = parent;
+                }
+            }
+        };
+        // Compress the path
+        while let Some(val) = val_stack.pop() {
             self.union_find[val] = UFEntry::encode_link(found.0);
         }
         found
     }
-        }
-    }
 
     /// Union the two sets containing `a` and `b`.
     ///