[Interpreter] Preserve source positions in peephole optimizer.

test/cctest/interpreter/bytecode_expectations/ThisFunction.golden

 #
 # Autogenerated by generate-bytecode-expectations.
 #
 
 ---
 pool type: number
 execute: yes
 wrap: no
 test function name: f
 
@@ skipping 19 matching lines @@
 ]
 
 ---
 snippet: "
   var f;
   f = function f() { return f; };
   f();
 "
 frame size: 1
 parameter count: 1
-bytecode array length: 6
+bytecode array length: 7
 bytecodes: [
   /*   21 E> */ B(StackCheck),
                 B(Ldar), R(closure),
                 B(Star), R(0),
+  /*   26 S> */ B(Nop),
   /*   36 S> */ B(Return),
 ]
 constant pool: [
 ]
 handlers: [
 ]