abc9: break conflict between boxes and outputs

tests/various/bug3311.ys

@@ -0,0 +1,29 @@
+read_rtlil <<EOT
+# Generated by Yosys 0.16+61 (git sha1 3730db4b9, clang 13.0.1 -fPIC -Os)
+autoidx 16887
+attribute \blackbox 1
+module \TRELLIS_IO
+  parameter \DIR
+  wire inout 1 \B
+  wire input 2 \I
+  wire output 3 \O
+  wire input 4 \T
+end
+attribute \top 1
+module \icesugarpro_top
+  wire width 8 inout 1 \P5_pmod_low
+  attribute \hdlname "vt spi 1796"
+  cell $_DFF_PP0_ $auto$ff.cc:262:slice$16873
+    connect \C 1'x
+    connect \D 1'x
+    connect \Q \P5_pmod_low [3]
+    connect \R 1'x
+  end
+  connect \P5_pmod_low [7:4] 4'x
+end
+EOT
+
+read_verilog -lib -specify +/ecp5/cells_sim.v
+techmap -map +/ecp5/cells_map.v
+abc9
+