Use BTreeMap to store block IDs

compiler/noirc_evaluator/src/ssa/opt/constant_folding.rs

@@ -196,7 +196,9 @@ pub(crate) struct BrilligInfo<'a> {
 /// For more information see [`ConstraintSimplificationCache`].
 #[derive(Default)]
 struct SimplificationCache {
-    simplifications: HashMap<BasicBlockId, ValueId>,
+    /// Simplified expressions where we found them.
+    /// Using a `BTreeMap` for deterministic enumeration.
+    simplifications: BTreeMap<BasicBlockId, ValueId>,
 }
 
 impl SimplificationCache {
@@ -221,7 +223,9 @@ impl SimplificationCache {
             return Some(*value);
         }
         // Check if there is a dominating block we can take a simplification from.
-        for (constraining_block, value) in self.simplifications.iter() {
+        // Going backwards so that we find a constraint closest to what we have already processed
+        // (assuming block IDs of blocks further down in the SSA are larger).
+        for (constraining_block, value) in self.simplifications.iter().rev() {
             if dom.dominates(*constraining_block, block) {
                 return Some(*value);
             }