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/ir/dfg.rs

@@ -323,6 +323,16 @@ pub enum ValueDef {
     Arg(Ebb, usize),
 }
 
+impl ValueDef {
+    /// Unwrap the instruction where the value was defined, or panic.
+    pub fn unwrap_inst(&self) -> Inst {
+        match self {
+            &ValueDef::Res(inst, _) => inst,
+            _ => panic!("Value is not an instruction result"),
+        }
+    }
+}
+
 // Internal table storage for extended values.
 #[derive(Clone, Debug)]
 enum ValueData {