Virtual registers.

Add a VirtRegs collection which tracks virtual registers.

A virtual register is a set of related SSA values whose live ranges
don't interfere. It is advantageous to use the same register or spill
slot for al the values in a virtual register. It reduces copies for EBB
arguments.

lib/cretonne/src/regalloc/mod.rs

@@ -15,6 +15,7 @@ mod pressure;
 mod reload;
 mod solver;
 mod spilling;
+mod virtregs;
 
 pub use self::allocatable_set::AllocatableSet;
 pub use self::context::Context;