use instructiont::turn_into_skip

This is a clearer idiom.

Author: Daniel Kroening

jbmc/src/java_bytecode/remove_instanceof.cpp

@@ -211,7 +211,7 @@ bool remove_instanceoft::lower_instanceof(
     // GOTO programs before the target instruction without inserting into the
     // wrong basic block.
     goto_program.insert_before_swap(target);
-    *target = goto_programt::make_skip();
+    target->turn_into_skip();
     // Actually alter the now-moved instruction:
     ++target;
   }

src/goto-instrument/accelerate/disjunctive_polynomial_acceleration.cpp

@@ -914,7 +914,7 @@ void disjunctive_polynomial_accelerationt::build_fixed()
     if(loop.find(t)==loop.end())
     {
       // This instruction isn't part of the loop...  Just remove it.
-      *fixedt = goto_programt::make_skip(fixedt->source_location);
+      fixedt->turn_into_skip();
       continue;
     }
 

src/goto-instrument/accelerate/sat_path_enumerator.cpp

@@ -269,7 +269,7 @@ void sat_path_enumeratort::build_fixed()
     if(loop.find(t)==loop.end())
     {
       // This instruction isn't part of the loop...  Just remove it.
-      *fixedt = goto_programt::make_skip(fixedt->source_location);
+      fixedt->turn_into_skip();
       continue;
     }