Remove regs from MachineEnv

It isn't exactly clear what purpose it serves.

src/fuzzing/func.rs

@@ -589,7 +589,6 @@ pub fn machine_env() -> MachineEnv {
         [regs.iter().cloned().skip(24).collect(), vec![]];
     let scratch_by_class: [PReg; 2] = [PReg::new(31, RegClass::Int), PReg::new(0, RegClass::Float)];
     MachineEnv {
-        regs,
         preferred_regs_by_class,
         non_preferred_regs_by_class,
         scratch_by_class,

src/ion/liveranges.rs

@@ -105,8 +105,11 @@ impl<'a, F: Function> Env<'a, F> {
                 allocations: LiveRangeSet::new(),
             },
         );
-        for &preg in &self.env.regs {
-            self.pregs[preg.index()].reg = preg;
+        for i in 0..=PReg::MAX {
+            let preg_int = PReg::new(i, RegClass::Int);
+            self.pregs[preg_int.index()].reg = preg_int;
+            let preg_float = PReg::new(i, RegClass::Float);
+            self.pregs[preg_float.index()].reg = preg_float;
         }
         // Create VRegs from the vreg count.
         for idx in 0..self.func.num_vregs() {

src/lib.rs

@@ -1139,12 +1139,6 @@ pub enum Edit {
 /// as well.
 #[derive(Clone, Debug)]
 pub struct MachineEnv {
-    /// Physical registers. Every register that might be mentioned in
-    /// any constraint must be listed here, even if it is not
-    /// allocatable (present in one of
-    /// `{preferred,non_preferred}_regs_by_class`).
-    pub regs: Vec<PReg>,
-
     /// Preferred physical registers for each class. These are the
     /// registers that will be allocated first, if free.
     pub preferred_regs_by_class: [Vec<PReg>; 2],