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".

cranelift/src/filetest/binemit.rs

@@ -126,7 +126,7 @@ impl SubTest for TestBinEmit {
         // Fix the stack frame layout so we can test spill/fill encodings.
         let min_offset = func.stack_slots
             .keys()
-            .map(|ss| func.stack_slots[ss].offset)
+            .map(|ss| func.stack_slots[ss].offset.unwrap())
             .min();
         func.stack_slots.frame_size = min_offset.map(|off| (-off) as u32);