Add support for fixed stack slots

This works by allowing a PReg to be marked as being a stack location
instead of a physical register.

src/ion/liveranges.rs

@@ -102,6 +102,7 @@ impl<'a, F: Function> Env<'a, F> {
             PRegData {
                 reg: PReg::invalid(),
                 allocations: LiveRangeSet::new(),
+                is_stack: false,
             },
         );
         for i in 0..=PReg::MAX {
@@ -110,6 +111,9 @@ impl<'a, F: Function> Env<'a, F> {
             let preg_float = PReg::new(i, RegClass::Float);
             self.pregs[preg_float.index()].reg = preg_float;
         }
+        for &preg in &self.env.fixed_stack_slots {
+            self.pregs[preg.index()].is_stack = true;
+        }
         // Create VRegs from the vreg count.
         for idx in 0..self.func.num_vregs() {
             // We'll fill in the real details when we see the def.