Fixed and simplified expression for lzc_a_input in serdiv

Created by: Seek64

Issue

The formal tool OneSpin 360 DV does not correctly handle the unsigned vs. signed comparison in op_a_i == -$signed(1), as it expects a $signed keyword for op_a_i. The commit fc2967cc added this extra check to correct a bug that occurs when op_a_i == -1. However, I then realized that this expression can be simplified.

Fix

Instead of appending 1'b1 if op_a_i == -1 and 0'b0 else, the functionality remains the same if 1'b1 is always added. The reason for this is that the last bit does not matter in the count leading zero operation whenever op_a_i != -1. I also added [$high(op_a_i)-1:0] to avoid the implicit truncation.

Example

Assume a WIDTH of 8: If op_a_i == -1 (8'b11111111) then lzc_a_input == 8'b00000001 and thus lzc_a_result == 7. If op_a_i == -2 (8'b11111110) then lzc_a_input == 8'b0000001X and thus lzc_a_result == 6. If op_a_i == -3 (8'b11111101) then lzc_a_input == 8'b0000010X and thus lzc_a_result == 5. And so on...