Remove register class from SpillSlot (#80)

* Remove register class from `SpillSlot`

The register allocator was already allowing moves between spillslots and
registers of different classes, so this PR formalizes this by making
spillslots independent of register class.

This also fixes #79 by properly tracking the register class of an
`InsertedMove` with the `to_vreg` field which turns out to never
be `None` in practice. Removing the `Option` allows the register
class of the `VReg` to be used when building the per-class move lists.

Fixes #79

* Address review feedback

src/ion/data_structures.rs

@@ -440,7 +440,7 @@ impl<'a, F: Function> Env<'a, F> {
 #[derive(Clone, Debug)]
 pub struct SpillSlotData {
     pub ranges: LiveRangeSet,
-    pub class: RegClass,
+    pub slots: u32,
     pub alloc: Allocation,
 }
 
@@ -580,7 +580,7 @@ pub struct InsertedMove {
     pub pos_prio: PosWithPrio,
     pub from_alloc: Allocation,
     pub to_alloc: Allocation,
-    pub to_vreg: Option<VReg>,
+    pub to_vreg: VReg,
 }
 
 #[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord)]