Include emergency stack slots when laying out the stack.

Emergency stack slots are a new kind of stack slot added relatively
recently. They need to be allocated a stack offset just like explicit
and spill slots.

Also, make StackSlotData's offset field an Option, to catch problems
like this in the future. Previously the value 0 was used when offsets
weren't assigned yet, however that made it non-obvious when the field
meant "not assigned yet" and when it meant "assigned the value 0".

lib/cretonne/src/verifier/mod.rs

@@ -808,7 +808,7 @@ impl<'a> Verifier<'a> {
                             slot
                         );
                     }
-                    if slot.offset != offset {
+                    if slot.offset != Some(offset) {
                         return err!(
                             inst,
                             "Outgoing stack argument {} should have offset {}: {} = {}",