[interpreter] Don't keep postfix result if only visited for effect

test/cctest/interpreter/bytecode_expectations/AssignmentsInBinaryExpression.golden

 #
 # Autogenerated by generate-bytecode-expectations.
 #
 
 ---
 pool type: string
 execute: yes
 wrap: yes
 
 ---
@@ skipping 81 matching lines @@
 
 ---
 snippet: "
   var x = 55;
   x = (x = 56) - x + (x = 57);
   x++;
   return x;
 "
 frame size: 3
 parameter count: 1
-bytecode array length: 28
+bytecode array length: 26
 bytecodes: [
   /*   30 E> */ B(StackCheck),
   /*   42 S> */ B(LdaSmi), U8(55),
                 B(Star), R(0),
   /*   46 S> */ B(LdaSmi), U8(56),
                 B(Star), R(0),
   /*   61 E> */ B(Sub), R(0),
                 B(Star), R(2),
                 B(LdaSmi), U8(57),
                 B(Star), R(0),
   /*   68 E> */ B(Add), R(2),
                 B(Star), R(0),
-  /*   75 S> */ B(ToNumber), R(1),
-                B(Inc),
+  /*   75 S> */ B(Inc),
                 B(Star), R(0),
   /*   80 S> */ B(Nop),
   /*   90 S> */ B(Return),
 ]
 constant pool: [
 ]
 handlers: [
 ]
 
 ---
@@ skipping 139 matching lines @@
                 B(Inc),
                 B(Star), R(0),
   /*   72 E> */ B(Add), R(3),
   /*   76 S> */ B(Return),
 ]
 constant pool: [
 ]
 handlers: [
 ]