[NFC][regalloc] type LiveInterval::reg() as Register

[mtrofin]

Summary

We have the Register type which precisely captures the role of this
member. Storage-wise, it's an unsigned.

This helps readability & maintainability.

[wmi]

LGTM.

llvm/include/llvm/CodeGen/LiveInterval.h

 #ifndef LLVM_CODEGEN_LIVEINTERVAL_H
 #define LLVM_CODEGEN_LIVEINTERVAL_H
 
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/IntEqClasses.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/iterator_range.h"
+#include "llvm/CodeGen/Register.h"
 #include "llvm/CodeGen/SlotIndexes.h"
 #include "llvm/MC/LaneBitmask.h"
 #include "llvm/Support/Allocator.h"
 #include "llvm/Support/MathExtras.h"
 #include <algorithm>
 #include <cassert>
 #include <cstddef>
 #include <functional>
public:
 
       void print(raw_ostream &OS) const;
       void dump() const;
     };
 
   private:
     SubRange *SubRanges = nullptr; ///< Single linked list of subregister live
                                    /// ranges.
-    const unsigned Reg; // the register or stack slot of this interval.
+    const Register Reg; // the register or stack slot of this interval.
     float Weight = 0.0; // weight of this interval
 
   public:
-    unsigned reg() const { return Reg; }
+    Register reg() const { return Reg; }
     float weight() const { return Weight; }
     void incrementWeight(float Inc) { Weight += Inc; }
     void setWeight(float Value) { Weight = Value; }
 
     LiveInterval(unsigned Reg, float Weight) : Reg(Reg), Weight(Weight) {}
 
     ~LiveInterval() {
       clearSubRanges();