Implement separate data flow graph module.

The DFG keeps track of instruction definitions, values, and EBBs.

Store the primary definition of each instruction: Opcode and operands.
Track SSA values as either the result of an instruction or EBB arguments.

cranelift/src/libcretonne/lib.rs

@@ -13,6 +13,7 @@ pub mod immediates;
 pub mod entities;
 pub mod instructions;
 pub mod repr;
+pub mod dfg;
 pub mod layout;
 pub mod write;
 pub mod cfg;