[bitmanip] Remove redundant logic

[ESE-2019]

Remove the redundant logic & ~bitcnt_mask_op, as explained in issue #2294 .

    always_comb begin
      unique case (1'b1)
        zbe_op:      bitcnt_bits = operand_b_i;
        // bitcnt_cz:   bitcnt_bits = bitcnt_bit_mask & ~bitcnt_mask_op; // clz / ctz
        bitcnt_cz:   bitcnt_bits = bitcnt_bit_mask; // clz / ctz
        default:     bitcnt_bits = operand_a_i; // cpop
      endcase
    end

[rswarbrick]

I think this is a sensible change, thanks. Would you mind expanding the commit message to explain what's going on? It's always easier to understand the history of the repository if it's not dependant on GitHub. With my "maths hat on", I'm always enthusiastic about giving a minimal "proof" of why a commit is the right thing to do.

Working out how to write such a commit message sometimes makes me adjust the code as well, which I think is probably a good thing. I'd also be tempted to extend this commit slightly to get rid of the slightly silly loops for replicating the bit. Is that easy? (And does it work? I only suggested this from reading the existing code...)