back.rtlil: fix handling of certain nested arrays.

back.rtlil: fix handling of certain nested arrays.

This triggers on code like:

c1 = Signal()
c2 = Signal()
c3 = Signal()
v1 = Array([Const(1, 8), Const(2, 8)])[c1]
v2 = Array([Const(3, 8), Const(4, 8)])[c2]
v3 = Array([v1, v2])[c3]

Fixes #226.