added instructiont::turn_into_skip()

This is an implementation for the common idiom in which an existing
instruction is replaced by a skip while retaining the source_location.  It
replaces instructiont::make_skip().

Author: Daniel Kroening

src/goto-programs/goto_program.h

@@ -332,6 +332,13 @@ class goto_programt
       code.make_nil();
     }
 
+    /// Transforms an existing instruction into a skip,
+    /// retaining the source_location
+    void turn_into_skip()
+    {
+      clear(SKIP);
+    }
+
     void make_return() { clear(RETURN); }
     void make_skip() { clear(SKIP); }
     void make_location(const source_locationt &l)