opt_lut: new pass, to combine LUTs for tighter packing.

tests/opt/opt_lut.v

@@ -0,0 +1,18 @@
+module top(
+    input [8:0] a,
+    input [8:0] b,
+    output [8:0] o1,
+    output [2:0] o2,
+    input [2:0] c,
+    input [2:0] d,
+    output [2:0] o3,
+    output [2:0] o4,
+    input s
+);
+
+assign o1 = (s ? 0 : a + b);
+assign o2 = (s ? a : a - b);
+assign o3 = (s ? 4'b1111 : d + c);
+assign o4 = (s ? d : c - d);
+
+endmodule