opt_lut: reflect changes in sigmap.

Otherwise, some LUTs will be missed during elimination.

passes/opt/opt_lut.cc

diff --git a/passes/opt/opt_lut.cc b/passes/opt/opt_lut.cc
index a79a9a2..b9a1ce7 100644
--- a/passes/opt/opt_lut.cc
+++ b/passes/opt/opt_lut.cc
@@ -271,6 +271,8 @@
 					}
 
 					module->connect(lut_output, value);
+					sigmap.add(lut_output, value);
+
 					module->remove(lut);
 					luts.erase(lut);
 					luts_arity.erase(lut);