[Interpreter] Preserve source positions in peephole optimizer.

test/mjsunit/ignition/elided-instruction.js

 // Copyright 2016 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// Flags: --ignition --expose-debug-as debug
+// Flags: --expose-debug-as debug
 
 Debug = debug.Debug
 
 var exception = null;
 var break_count = 0;
 
 function listener(event, exec_state, event_data, data) {
   if (event != Debug.DebugEvent.Break) return;
   try {
     print(event_data.sourceLineText());
     var column = event_data.sourceColumn();
     assertTrue(event_data.sourceLineText().indexOf(
         `Break ${break_count++}. ${column}.`) > 0);
     exec_state.prepareStep(Debug.StepAction.StepIn);
   } catch (e) {
     print(e + e.stack);
     exception = e;
   }
 };
 
 function f() {
   var a = 1;                     // Break 2. 10.
-  // This return statement emits no bytecode instruction for the evaluation of
-  // the to-be-returned expression. Therefore we cannot set a break location
-  // before the statement and a second break location immediately before
-  // returning to the caller.
-  return a;
-}                                // Break 3. 0.
+  return a;                      // Break 3. 2.
+}                                // Break 4. 0.
 
 Debug.setListener(listener);
 debugger;                        // Break 0. 0.
 f();                             // Break 1. 0.
-Debug.setListener(null);         // Break 4. 0.
+Debug.setListener(null);         // Break 5. 0.
 
 assertNull(exception);
-assertEquals(5, break_count);
+assertEquals(6, break_count);