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/pressure.rs

@@ -39,6 +39,7 @@
 use isa::registers::{RegInfo, MAX_TOPRCS, RegClass, RegClassMask};
 use regalloc::AllocatableSet;
 use std::cmp::min;
+use std::fmt;
 use std::iter::ExactSizeIterator;
 
 /// Information per top-level register class.
@@ -225,6 +226,16 @@ impl Pressure {
     }
 }
 
+impl fmt::Display for Pressure {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "Pressure[")?;
+        for rc in &self.toprc {
+            write!(f, " {}+{}/{}", rc.base_count, rc.transient_count, rc.limit)?;
+        }
+        write!(f, " ]")
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use isa::{TargetIsa, RegClass};