memory_dff: Fix needlessly duplicating enable bits.

When the register being merged into the EN signal happens to be a $sdff,
the current code creates a new $mux for every bit, even if they happen
to be identical (as is usually the case), preventing proper grouping
further down the flow.  Fix this by adding a simple cache.

Fixes #2409.

tests/arch/ecp5/bug2409.ys

@@ -0,0 +1,24 @@
+read_verilog <<EOT
+module t (...);
+
+input CLK;
+input [10:0] A;
+input WE;
+input C;
+input [7:0] DI;
+output reg [7:0] DO;
+
+reg [7:0] mem[2047:0];
+
+always @(posedge CLK) begin
+	if (C)
+		if (WE)
+			mem[A] <= DI;
+	DO <= mem[A];
+end
+
+endmodule
+EOT
+
+synth_ecp5
+select -assert-count 1 t:DP16KD