Add Rebroadcast to the list of ops get_constant_value digs through.

theano/tensor/opt.py

@@ -87,6 +87,8 @@ def get_constant_value(v):
             return get_constant_value(v.owner.inputs[0])
         if isinstance(v.owner.op, T.DimShuffle):
             return get_constant_value(v.owner.inputs[0])
+        if isinstance(v.owner.op, T.Rebroadcast):
+            return get_constant_value(v.owner.inputs[0]))
         if v.owner.op == T.fill:
             shape, val = v.owner.inputs
             # fill(a,b) fills the shape of 'a' filled with 'b'