hdl.ir: accept LHS signals like slices as Instance io ports.

This is unlikely to work with anything except Slice and Cat, but
there's no especially good place to enforce it. (Maybe in Instance?)

nmigen/hdl/ir.py

@@ -373,9 +373,12 @@ class Fragment:
                 else:
                     assert defs[sig] is self
 
-        def add_io(sig):
+        def add_io(*sigs):
-            assert sig not in ios
+            for sig in flatten(sigs):
-            ios[sig] = self
+                if sig not in ios:
+                    ios[sig] = self
+                else:
+                    assert ios[sig] is self
 
         # Collect all signals we're driving (on LHS of statements), and signals we're using
         # (on RHS of statements, or in clock domains).
@@ -400,8 +403,8 @@ class Fragment:
                         subfrag.add_ports(value._lhs_signals(), dir=dir)
                         add_defs(value._lhs_signals())
                     if dir == "io":
-                        subfrag.add_ports(value, dir=dir)
+                        subfrag.add_ports(value._lhs_signals(), dir=dir)
-                        add_io(value)
+                        add_io(value._lhs_signals())
             else:
                 parent[subfrag] = self
                 level [subfrag] = level[self] + 1

nmigen/test/test_hdl_ir.py

@@ -603,7 +603,7 @@ class InstanceTestCase(FHDLTestCase):
             i_rst=self.rst,
             o_stb=self.stb,
             o_data=Cat(self.datal, self.datah),
-            io_pins=self.pins
+            io_pins=self.pins[:]
         )
         self.wrap = Fragment()
         self.wrap.add_subfragment(self.inst)