Fork fission bufferize

• Refactor fork fission, add fork fission bufferized version.
• Add test that uses fork fission to create extra forks.

hercules_opt/src/forkify.rs

@@ -152,6 +152,7 @@ pub fn forkify_loop(
         .filter(|id| !l.control[id.idx()])
         .collect();
 
+    // FIXME: @xrouth
     if loop_preds.len() != 1 {
         return false;
     }
@@ -388,6 +389,7 @@ nest! {
             is_associative: bool,
         },
         LoopDependant(NodeID),
+        ControlDependant(NodeID), // This phi is redcutionable, but its cycle might depend on internal control within the loop.
         UsedByDependant(NodeID),
     }
 }
@@ -398,6 +400,7 @@ impl LoopPHI {
             LoopPHI::Reductionable { phi, .. } => *phi,
             LoopPHI::LoopDependant(node_id) => *node_id,
             LoopPHI::UsedByDependant(node_id) => *node_id,
+            LoopPHI::ControlDependant(node_id) => *node_id,
         }
     }
 }
@@ -415,6 +418,9 @@ pub fn analyze_phis<'a>(
     loop_nodes: &'a HashSet<NodeID>,
 ) -> impl Iterator<Item = LoopPHI> + 'a {
 
+    // We are also moving the phi from the top of the loop (the header),
+    // to the very end (the join). If there are uses of the phi somewhere in the loop,
+    // then they may try to use the phi (now a reduce) before it hits the join. 
     // Find data cycles within the loop of this phi, 
     // Start from the phis loop_continue_latch, and walk its uses until we find the original phi. 
 
@@ -509,6 +515,12 @@ pub fn analyze_phis<'a>(
             // to have headers that postdominate the loop continue latch. The value of the PHI used needs to be defined
             // by the time the reduce is triggered (at the end of the loop's internal control).
 
+            // If anything in the intersection is a phi (that isn't this own phi), then the reduction cycle depends on control.
+            // Which is not allowed.
+            if intersection.iter().any(|cycle_node| editor.node(cycle_node).is_phi() && *cycle_node != *phi) || editor.node(loop_continue_latch).is_phi() {
+                return LoopPHI::ControlDependant(*phi);
+            }
+
             // No nodes in data cycles with this phi (in the loop) are used outside the loop, besides the loop_continue_latch.
             // If some other node in the cycle is used, there is not a valid node to assign it after making the cycle a reduce.
             if intersection