Allow LocalVariable to be assigned more than once (#2288)

* Allow `LocalVariable` to be assigned more than once

This allows us to write flow controls like loops and if-elses with
LocalVariables participating in phi nodes.

* Add `GetLocalNumber` to operand

ARMeilleure/IntermediateRepresentation/Operand.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.Diagnostics;
 using System.Runtime.CompilerServices;
 
 namespace ARMeilleure.IntermediateRepresentation
@@ -91,6 +92,13 @@ namespace ARMeilleure.IntermediateRepresentation
             return new Register((int)Value & 0xffffff, (RegisterType)(Value >> 24));
         }
 
+        public int GetLocalNumber()
+        {
+            Debug.Assert(Kind == OperandKind.LocalVariable);
+
+            return (int)Value;
+        }
+
         public byte AsByte()
         {
             return (byte)Value;