Fix corner case of pos cell with input and output being same width

tests/functional/single_cells/rtlil/test_cell_pos_00001.il

@@ -0,0 +1,13 @@
+# Generated by Yosys 0.40+7 (git sha1 d8b5b90e7, g++ 13.2.0 -Og -fPIC)
+autoidx 1
+module \gold
+  wire width 6 input 1 \A
+  wire width 6 output 2 \Y
+  cell $pos \UUT
+    parameter \A_SIGNED 0
+    parameter \A_WIDTH 6
+    parameter \Y_WIDTH 6
+    connect \A \A
+    connect \Y \Y
+  end
+end