Add a minimalistic reload pass.

The reload pass inserts spill and fill instructions as needed so
instructions that operate on registers will never see a value with stack
affinity.

This is a very basic implementation, and we can't write good test cases
until we have a spilling pass.

lib/cretonne/src/regalloc/mod.rs

@@ -12,6 +12,7 @@ mod affinity;
 mod context;
 mod diversion;
 mod pressure;
+mod reload;
 mod solver;
 
 pub use self::allocatable_set::AllocatableSet;