+--
+-- SPILL r0 -> s1
+-- RELOAD s1 -> r2
+-- SPILL r3 -> s1 <--- don't need this spill
+-- SPILL r4 -> s1
+-- RELOAD s1 -> r5
+--
+-- Maintain a set of
+-- "slots which were spilled to but not reloaded from yet"
+--
+-- Walking backwards across the code:
+-- a) On a reload from a slot, remove it from the set.
+--
+-- a) On a spill from a slot
+-- If the slot is in set then we can erase the spill,
+-- because it won't be reloaded from until after the next spill.
+--
+-- otherwise
+-- keep the spill and add the slot to the set
+--
+-- TODO: This is mostly inter-block
+-- we should really be updating the noReloads set as we cross jumps also.
+--
+cleanBackward