Expose killed_remat_defs publicly in regalloc assignment

Currently, the only way to ask about killed remat defs using the public
API is to construct an edit tracker, but there's no reason a user
shouldn't be able to just access the information directly.

Author: nmraz

crates/codegen/src/regalloc.rs

@@ -57,9 +57,9 @@ pub struct Assignment {
     pub spill_slots: PrimaryMap<SpillSlot, SpillSlotData>,
     pub copies: Vec<TaggedAssignmentCopy>,
     pub block_exit_ghost_copies: Vec<BlockExitGhostCopy>,
+    pub killed_remat_defs: DenseEntitySet<Instr>,
     instr_assignments: SecondaryMap<Instr, InstrAssignmentData>,
     operand_assignment_pool: Vec<OperandAssignment>,
-    killed_remat_defs: DenseEntitySet<Instr>,
 }
 
 impl Assignment {