lzc: Fix edge case of WIDTH==0 (#266)

* [CIRCT] Fix edge case of WIDTH == 0 routed into degenerate case

* Move assert to before case distinction, remove redundant assert

Author: MoritzScherer

src/lzc.sv

@@ -32,7 +32,11 @@ module lzc #(
   output logic                 empty_o
 );
 
-  if (WIDTH == 1) begin : gen_degenerate_lzc
+  `ifndef COMMON_CELLS_ASSERTS_OFF
+    `ASSERT_INIT(width_0, WIDTH > 0, "input must be at least one bit wide")
+  `endif
+
+  if (WIDTH <= 1) begin : gen_degenerate_lzc
 
     assign cnt_o[0] = !in_i[0];
     assign empty_o = !in_i[0];
@@ -41,10 +45,6 @@ module lzc #(
 
     localparam int unsigned NumLevels = $clog2(WIDTH);
 
-  `ifndef COMMON_CELLS_ASSERTS_OFF
-    `ASSERT_INIT(width_0, WIDTH > 0, "input must be at least one bit wide")
-  `endif
-
     logic [WIDTH-1:0][NumLevels-1:0] index_lut;
     logic [2**NumLevels-1:0] sel_nodes                  /* verilator split_var */;
     logic [2**NumLevels-1:0][NumLevels-1:0] index_nodes /* verilator split_var */;
@@ -104,8 +104,4 @@ module lzc #(
 
   end : gen_lzc
 
-`ifndef COMMON_CELLS_ASSERTS_OFF
-  `ASSERT_INIT(width_0, WIDTH >= 1, "The WIDTH must at least be one bit wide!")
-`endif
-
 endmodule : lzc