Make sure push_ready[i] not dependent on push_valid[i] in br_flow_arb libraries

[zhemao-openai]

The various flow arbiter implementations have the following logic ostensibly to make an individual bit in push_ready not depend on its own push_valid.

  for (genvar i = 0; i < NumFlows; i++) begin : gen_push_ready
    always_comb begin
      push_ready[i] = pop_ready;
      for (int j = 0; j < NumFlows; j++) begin
        if (i != j) begin
          push_ready[i] &= !grant[j];
        end
      end
    end
  end

However, this doesn’t actually accomplish it’s stated goal, since the grant[j] might depend on push_valid[i] even if i != j. Besides this, it creates a lot of additional logic that causes timing issues. The proper way to fix this is to create internal versions of the base arbiter modules that expose a “can_grant” signal for which can_grant[i] is true if no higher priority request is asserted. The request, grant, and can_grant signals should meet the invariant grant == (request & can_grant).