Remove PRegData::reg and use PReg::from_index instead

Performance impact is negligible but this is a good cleanup.

src/ion/data_structures.rs

@@ -264,7 +264,6 @@ pub struct VRegData {
 
 #[derive(Clone, Debug)]
 pub struct PRegData {
-    pub reg: PReg,
     pub allocations: LiveRangeSet,
     pub is_stack: bool,
 }