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.

cranelift/filetests/parser/tiny.cton

@@ -117,6 +117,7 @@ function %stack() {
     ss2 = local 4
     ss3 = incoming_arg 4, offset 8
     ss4 = outgoing_arg 4
+    ss5 = emergency_slot 4
 
 ebb0:
     v1 = stack_load.i32 ss10
@@ -129,6 +130,7 @@ ebb0:
 ; nextln:    $ss2 = local 4
 ; nextln:    $ss3 = incoming_arg 4, offset 8
 ; nextln:    $ss4 = outgoing_arg 4
+; nextln:    $ss5 = emergency_slot 4
 
 ; check: ebb0:
 ; nextln: $v1 = stack_load.i32 $ss10