[NFC] ConstraintAnalysis: Avoid a copy when there are no interesting branch constraints

src/passes/ConstraintAnalysis.cpp

@@ -165,14 +165,19 @@ struct ConstraintAnalysis
 
         // Find the constraints sent to this specific successor, if there is a
         // branch, and use them.
-        auto sentConstraints = constraints;
         if (auto branch = getBranchConstraints(block, out)) {
+          auto sentConstraints = constraints;
           sentConstraints.approximateAnd(branch->local, branch->constraint);
-        }
-
-        // If anything changed at the start of the target block, flow onwards.
-        if (outStartConstraints.approximateOr(sentConstraints)) {
-          work.push(out);
+          // If anything changed at the start of the target block, flow onwards.
+          if (outStartConstraints.approximateOr(sentConstraints)) {
+            work.push(out);
+          }
+        } else {
+          // There are no specific branch constraints, so send the unmodified
+          // |constraints|, avoiding a copy.
+          if (outStartConstraints.approximateOr(constraints)) {
+            work.push(out);
+          }
         }
       }
     }