compiler: fix overly strict constness analysis.

Before this commit, the following code would fail to compile...
  obj.foo.bar = True
... if foo is marked kernel_invariant in obj, even if bar is not
marked as such in obj.foo.

artiq/compiler/analyses/constness.py

@@ -24,7 +24,10 @@ class Constness(algorithm.Visitor):
         self.in_assign = old_in_assign
 
     def visit_AttributeT(self, node):
-        self.generic_visit(node)
+        old_in_assign, self.in_assign = self.in_assign, False
+        self.visit(node.value)
+        self.in_assign = old_in_assign
+
         if self.in_assign:
             typ = node.value.type.find()
             if types.is_instance(typ) and node.attr in typ.constant_attributes:

artiq/test/lit/embedding/invariant_nested.py

@@ -0,0 +1,22 @@
+# RUN: %python -m artiq.compiler.testbench.embedding %s
+
+from artiq.language.core import *
+from artiq.language.types import *
+
+
+class ClassA:
+    def __init__(self):
+        self.foo = False
+
+
+class ClassB:
+    kernel_invariants = {"bar"}
+
+    def __init__(self):
+        self.bar = ClassA()
+
+obj = ClassB()
+
+@kernel
+def entrypoint():
+    obj.bar.foo = True