Basic spilling implementation.

Add a spilling pass which lowers register pressure by assigning SSA
values to the stack. Important missing features:

- Resolve conflicts where an instruction uses the same value more than
  once in incompatible ways.
- Deal with EBB arguments.

Fix bugs in the reload pass exposed by the first test case:

- Create live ranges for temporary registers.
- Set encodings on created spill and fill instructions.

lib/cretonne/src/regalloc/live_value_tracker.rs

@@ -12,7 +12,6 @@ use partition_slice::partition_slice;
 use regalloc::affinity::Affinity;
 use regalloc::liveness::Liveness;
 use regalloc::liverange::LiveRange;
-
 use std::collections::HashMap;
 
 type ValueList = EntityList<Value>;
@@ -299,6 +298,20 @@ impl LiveValueTracker {
         self.live.remove_dead_values();
     }
 
+    /// Process new spills.
+    ///
+    /// Any values where `f` returns true are spilled and will be treated as if their affinity was
+    /// `Stack`.
+    pub fn process_spills<F>(&mut self, mut f: F)
+        where F: FnMut(Value) -> bool
+    {
+        for lv in &mut self.live.values {
+            if f(lv.value) {
+                lv.affinity = Affinity::Stack;
+            }
+        }
+    }
+
     /// Save the current set of live values so it is associated with `idom`.
     fn save_idom_live_set(&mut self, idom: Inst) {
         let values = self.live.values.iter().map(|lv| lv.value);