Implement opt_share

This pass identifies arithmetic operators that share an operand and whose
results are used in mutually exclusive cases controlled by a multiplexer, and
merges them together by multiplexing the other operands

tests/opt/opt_share_cat.v

@@ -0,0 +1,15 @@
+module add_sub(
+               input [15:0]  a,
+               input [15:0]  b,
+               input [15:0]  c,
+               input [15:0]  d,
+               input 				 sel,
+               output [63:0] res,
+               );
+
+  reg [31: 0] 							 cat1 = {a+b, c+d};
+  reg [31: 0] 							 cat2 = {a-b, c-d};
+
+  assign res = {b, sel ? cat1 : cat2, a};
+
+endmodule

tests/opt/opt_share_cat.ys

@@ -0,0 +1,9 @@
+read_verilog opt_share_cat.v
+prep -flatten
+opt
+pmuxtree
+opt_share
+opt_clean
+
+select -assert-count 2 t:$sub
+select -assert-count 0 t:$add

tests/opt/opt_share_mux_tree.v

@@ -0,0 +1,19 @@
+module add_sub(
+               input [15:0] 		 a,
+               input [15:0] 		 b,
+               input [15:0] 		 c,
+               input [1:0] 			 sel,
+               output reg [15:0] res
+               );
+
+
+  always @* begin
+    case(sel)
+      0: res = a + b;
+      1: res = a - b;
+      2: res = a + c;
+      default: res = 16'bx;
+    endcase
+  end
+
+endmodule

tests/opt/opt_share_mux_tree.ys

@@ -0,0 +1,10 @@
+read_verilog opt_share_mux_tree.v
+prep -flatten
+opt
+pmuxtree
+opt_share;
+opt_share;
+opt_clean
+
+select -assert-count 1 t:$add
+select -assert-count 0 t:$sub