ecp5: Fix constant and inverted CEMUX

[gatecat]

In one case, Yosys generates a never-toggling flipflop with constant-1 CE and CEMUX set to INV. This is a bit odd, but nextpnr should still handle this case correctly.

[leocassarani]

@gatecat Thanks for fixing this so quickly! I've just tested your patch with the following repro case and I can confirm that it works as expected on my board (i.e. the LED stays off) — as opposed to nextpnr on master where the LED would be on. Much appreciated 🙏

module top (
  input clk,
  output reg led_g = 1'b0
);

  reg a = 1'b0;
  always @(posedge clk)
    a <= !a;

  wire [2:0] b = 3'd3 + { 2'b0, a };

  always @(posedge clk)
    if (b == 3'd0) led_g <= 1'b1;

endmodule