opt_clean: Remove init attribute bits together with removed DFFs.

Fixes #2546.

tests/opt/opt_clean_init.ys

@@ -1,13 +1,22 @@
-logger -expect warning "Initial value conflict for \\y resolving to 1'0 but with init 1'1" 1
-logger -expect-no-warnings
-read_verilog <<EOT
-module top;
-(* init=1'b0 *) wire w = 1'b0;
-(* init=1'bx *) wire x = 1'b0;
-(* init=1'b1 *) wire y = 1'b0;
-(* init=1'b0 *) wire z = 1'bx;
+read_verilog << EOT
+module top(...);
+
+input [1:0] D;
+input C;
+output O;
+reg [1:0] Q;
+
+initial Q = 0;
+
+always @(posedge C)
+        Q <= D;
+
+assign O = Q[1];
+
 endmodule
 EOT
-clean
-select -assert-count 1 a:init
-select -assert-count 1 w:y a:init %i
+
+synth
+check -assert -initdrv
+
+select -assert-count 1 a:init=2'b0x