back.rtlil: do not translate empty subfragments at all.

It was thought previously (by me) that adding a wire that does
nothing to an empty subfragment is enough to prevent it from being
treated as a blackbox. This is enough for Yosys but not Vivado.
Another workaround could probably be used that satisfies both, but
instead let's just not translate any empty subfragments.

This doesn't account for the case of the empty toplevel, but that
does not seem worth addressing.

Fixes #899.

amaranth/back/rtlil.py

@@ -832,11 +832,6 @@ def _convert_fragment(builder, fragment, name_map, hierarchy):
         lhs_compiler   = _LHSValueCompiler(compiler_state)
         stmt_compiler  = _StatementCompiler(compiler_state, rhs_compiler, lhs_compiler)
 
-        # If the fragment is completely empty, add a dummy wire to it, or Yosys will interpret
-        # it as a black box by default (when read as Verilog).
-        if not fragment.ports and not fragment.statements and not fragment.subfragments:
-            module.wire(1, name="$empty_module_filler")
-
         # Register all signals driven in the current fragment. This must be done first, as it
         # affects further codegen; e.g. whether \sig$next signals will be generated and used.
         for domain, signal in fragment.iter_drivers():
@@ -861,6 +856,12 @@ def _convert_fragment(builder, fragment, name_map, hierarchy):
         # name) names.
         memories = OrderedDict()
         for subfragment, sub_name in fragment.subfragments:
+            if not (subfragment.ports or subfragment.statements or subfragment.subfragments):
+                # If the fragment is completely empty, skip translating it, otherwise synthesis
+                # tools (including Yosys and Vivado) will treat it as a black box when it is
+                # loaded after conversion to Verilog.
+                continue
+
             if sub_name is None:
                 sub_name = module.anonymous()