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/liveness.rs

@@ -182,6 +182,7 @@ use isa::{TargetIsa, EncInfo};
 use regalloc::affinity::Affinity;
 use regalloc::liverange::LiveRange;
 use sparse_map::SparseMap;
+use std::mem;
 
 /// A set of live ranges, indexed by value number.
 type LiveRangeSet = SparseMap<Value, LiveRange>;
@@ -338,6 +339,13 @@ impl Liveness {
         &mut lr.affinity
     }
 
+    /// Change the affinity of `value` to `Stack` and return the previous affinity.
+    pub fn spill(&mut self, value: Value) -> Affinity {
+        let mut lr = self.ranges.get_mut(value).expect("Value has no live range");
+        mem::replace(&mut lr.affinity, Affinity::Stack)
+    }
+
+
     /// Compute the live ranges of all SSA values used in `func`.
     /// This clears out any existing analysis stored in this data structure.
     pub fn compute(&mut self, isa: &TargetIsa, func: &Function, cfg: &ControlFlowGraph) {