Add test for abc9+mince issue

Signed-off-by: David Shah <dave@ds0.me>

tests/various/ice40_mince_abc9.ys

@@ -0,0 +1,17 @@
+read_verilog <<EOT
+
+module top(input clk, ce, input [2:0] a, b, output reg [2:0] q);
+
+	reg [2:0] aa, bb;
+
+	always @(posedge clk) begin
+		if (ce) begin
+			aa <= a;
+		end
+		bb <= b;
+		q <= aa + bb;
+	end
+endmodule
+EOT
+
+synth_ice40 -abc9 -dffe_min_ce_use 4