Add support for emergency spill slots.

- Create a new kind of stack slot: emergency_slot.
- Add a get_emergency_slot() method which finds a suitable emergency
  slot given a list of slots already in use.
- Use emergency spill slots when schedule_moves needs them.

lib/cretonne/src/stack_layout.rs

@@ -56,7 +56,9 @@ pub fn layout_stack(frame: &mut StackSlots, alignment: StackSize) -> Result<Stac
                 )?;
                 outgoing_max = max(outgoing_max, offset);
             }
-            StackSlotKind::SpillSlot | StackSlotKind::Local => {
+            StackSlotKind::SpillSlot |
+            StackSlotKind::Local |
+            StackSlotKind::EmergencySlot => {
                 // Determine the smallest alignment of any local or spill slot.
                 min_align = slot.alignment(min_align);
             }