transforms.iodelay_estimator: correctly handle functions with empty body.

artiq/compiler/iodelay.py

@@ -208,6 +208,7 @@ class Max(Expr):
     def __init__(self, operands):
         assert isinstance(operands, list)
         assert all([isinstance(operand, Expr) for operand in operands])
+        assert operands != []
         self.operands = operands
 
     def __str__(self):

artiq/compiler/transforms/iodelay_estimator.py

@@ -220,7 +220,8 @@ class IODelayEstimator(algorithm.Visitor):
                     delays.append(self.current_delay)
                     self.current_delay = old_delay
 
-                self.current_delay += iodelay.Max(delays)
+                if any(delays):
+                    self.current_delay += iodelay.Max(delays)
             except _IndeterminateDelay as error:
                 # Interleave failures inside `with` statements are hard failures,
                 # since there's no chance that the code will never actually execute