machxo2: Switch to LUT4 sim model which propagates less undefined/don't care values.

techlibs/machxo2/cells_sim.v

@@ -4,17 +4,11 @@ module LUT4 #(
 	input A, B, C, D,
 	output Z
 );
-	wire [3:0] I;
-	wire [3:0] I_pd;
-
-	genvar ii;
-	generate
-		for (ii = 0; ii < 4; ii = ii + 1'b1)
-			assign I_pd[ii] = (I[ii] === 1'bz) ? 1'b0 : I[ii];
-	endgenerate
-
-	assign I = {D, C, B, A};
-	assign Z = INIT[I_pd];
+	// This form of LUT propagates as few x's as possible.
+	wire [7:0] s3 = D ?     INIT[15:8] :     INIT[7:0];
+	wire [3:0] s2 = C ?       s3[ 7:4] :       s3[3:0];
+	wire [1:0] s1 = B ?       s2[ 3:2] :       s2[1:0];
+	assign Z =      A ?          s1[1] :         s1[0];
 endmodule
 
 module FACADE_FF #(