Docs: working on opt page

Replace leftover `opt` example source/images with examples specific to the `opt_*` pass.
Currently has images for `opt_expr`, `opt_merge`, `opt_muxtree`, and `opt_share`.
Also includes some other TODO updates.

docs/source/code_examples/opt/Makefile

@@ -0,0 +1,19 @@
+PROGRAM_PREFIX :=
+
+YOSYS ?= ../../../../$(PROGRAM_PREFIX)yosys
+
+DOT_NAMES = opt_share opt_muxtree opt_merge opt_expr
+
+DOTS := $(addsuffix .dot,$(DOT_NAMES))
+
+dots: $(DOTS)
+
+%_full.dot: %.ys
+	$(YOSYS) $<
+
+%.dot: %_full.dot
+	gvpack -u $*_full.dot -o $@
+
+.PHONY: clean
+clean:
+	rm -f *.dot

docs/source/code_examples/opt/opt_expr.ys

@@ -0,0 +1,17 @@
+read_verilog <<EOT
+
+module uut(
+    input a,
+    output y, z
+);  
+    assign y = a == a;
+    assign z = a != a;
+endmodule
+
+EOT
+
+copy uut after
+opt_expr after
+clean
+
+show -format dot -prefix opt_expr_full -notitle -color cornflowerblue uut

docs/source/code_examples/opt/opt_merge.ys

@@ -0,0 +1,18 @@
+read_verilog <<EOT
+
+module uut(
+    input  [3:0] a, b,
+    output [3:0] y, z
+);
+    assign y = a + b;
+    assign z = b + a;
+endmodule
+
+EOT
+
+copy uut after
+opt_merge after
+clean
+
+show -format dot -prefix opt_merge_full -notitle -color cornflowerblue uut
+

docs/source/code_examples/opt/opt_muxtree.ys

@@ -0,0 +1,17 @@
+read_verilog <<EOT
+
+module uut(
+    input a, b, c, d,
+    output y
+);  
+    assign y = a ? (a ? b : c) : d;
+endmodule
+
+EOT
+
+copy uut after
+opt_muxtree after
+clean
+
+show -format dot -prefix opt_muxtree_full -notitle -color cornflowerblue uut
+

docs/source/code_examples/opt/opt_share.ys

@@ -0,0 +1,17 @@
+read_verilog <<EOT
+
+module uut(
+    input  [15:0] a, b,
+    input         sel,
+    output [15:0] res,
+);
+    assign res = {sel ? a + b : a - b};
+endmodule
+
+EOT
+
+copy uut after
+opt_share after
+clean
+
+show -format dot -prefix opt_share_full -notitle -color cornflowerblue uut