back.rtlil: only expand legalized values in Array/Part context on RHS.

Otherwise the following code fails to compile:

    index = Signal(1)
    array = Array(range(2))
    with m.If(0 == array[index]):
        m.d.sync += index.eq(0)

Fixes #51.

nmigen/back/rtlil.py

@@ -299,9 +299,6 @@ class _ValueCompiler(xfrm.ValueVisitor):
     def __init__(self, state):
         self.s = state
 
-    def on_value(self, value):
-        return super().on_value(self.s.expand(value))
-
     def on_unknown(self, value):
         if value is None:
             return None
@@ -375,6 +372,9 @@ class _RHSValueCompiler(_ValueCompiler):
         (3, "m"):    "$mux",
     }
 
+    def on_value(self, value):
+        return super().on_value(self.s.expand(value))
+
     def on_Const(self, value):
         if isinstance(value.value, str):
             return "{}'{}".format(value.nbits, value.value)