[Interpreter] Preserve source positions in peephole optimizer.

test/cctest/interpreter/bytecode_expectations/DoExpression.golden

 #
 # Autogenerated by generate-bytecode-expectations.
 #
 
 ---
 pool type: string
 execute: yes
 wrap: yes
 do expressions: yes
 
 ---
 snippet: "
   var a = do { }; return a;
 "
 frame size: 2
 parameter count: 1
-bytecode array length: 6
+bytecode array length: 7
 bytecodes: [
   /*   30 E> */ B(StackCheck),
   /*   42 S> */ B(Ldar), R(0),
   /*   42 E> */ B(Star), R(1),
+  /*   50 S> */ B(Nop),
   /*   60 S> */ B(Return),
 ]
 constant pool: [
 ]
 handlers: [
 ]
 
 ---
 snippet: "
   var a = do { var x = 100; }; return a;
 "
 frame size: 3
 parameter count: 1
-bytecode array length: 11
+bytecode array length: 12
 bytecodes: [
   /*   30 E> */ B(StackCheck),
   /*   55 S> */ B(LdaSmi), U8(100),
   /*   55 E> */ B(Star), R(1),
   /*   42 S> */ B(LdaUndefined),
                 B(Star), R(0),
   /*   42 E> */ B(Star), R(2),
+  /*   63 S> */ B(Nop),
   /*   73 S> */ B(Return),
 ]
 constant pool: [
 ]
 handlers: [
 ]
 
 ---
 snippet: "
   while(true) { var a = 10; a = do { ++a; break; }; a = 20; }
@@ skipping 16 matching lines @@
   /*   86 E> */ B(Star), R(1),
                 B(Jump), U8(-20),
                 B(LdaUndefined),
   /*   94 S> */ B(Return),
 ]
 constant pool: [
 ]
 handlers: [
 ]