Make instructions_equal static

Author: reuk

src/goto-diff/unified_diff.cpp

@@ -401,7 +401,7 @@ void unified_difft::output(std::ostream &os) const
 bool unified_difft::instructions_equal(
   const goto_programt::instructiont &ins1,
   const goto_programt::instructiont &ins2,
-  bool recurse) const
+  bool recurse)
 {
   return
     ins1.code==ins2.code &&

src/goto-diff/unified_diff.h

@@ -85,10 +85,10 @@ class unified_difft
     const differencest &differences,
     std::ostream &os) const;
 
-  bool instructions_equal(
+  static bool instructions_equal(
     const goto_programt::instructiont &ins1,
     const goto_programt::instructiont &ins2,
-    bool recurse=true) const;
+    bool recurse=true);
 
   const differences_mapt &differences_map() const;