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