[for-in] Ensure that we learn from deopts within for-in loop bodies.

src/full-codegen/x64/full-codegen-x64.cc

Index: src/full-codegen/x64/full-codegen-x64.cc
diff --git a/src/full-codegen/x64/full-codegen-x64.cc b/src/full-codegen/x64/full-codegen-x64.cc
index 790d9d65a4f0734aea80a9637dce968b998eadc2..264c298579a4f729fdf02ad67612c1dab6566138 100644
--- a/src/full-codegen/x64/full-codegen-x64.cc
+++ b/src/full-codegen/x64/full-codegen-x64.cc
@@ -1068,8 +1068,8 @@ void FullCodeGenerator::VisitForInStatement(ForInStatement* stmt) {
   __ bind(&fixed_array);
 
   // No need for a write barrier, we are storing a Smi in the feedback vector.
+  int const vector_index = SmiFromSlot(slot)->value();
   __ EmitLoadTypeFeedbackVector(rbx);
-  int vector_index = SmiFromSlot(slot)->value();
   __ Move(FieldOperand(rbx, FixedArray::OffsetOfElementAt(vector_index)),
           TypeFeedbackVector::MegamorphicSentinel(isolate()));
   __ movp(rcx, Operand(rsp, 0 * kPointerSize));  // Get enumerated object
@@ -1107,6 +1107,16 @@ void FullCodeGenerator::VisitForInStatement(ForInStatement* stmt) {
   __ cmpp(rdx, FieldOperand(rcx, HeapObject::kMapOffset));
   __ j(equal, &update_each, Label::kNear);
 
+  // We might get here from TurboFan or Crankshaft when something in the
+  // for-in loop body deopts and only now notice in fullcodegen, that we
+  // can now longer use the enum cache, i.e. left fast mode. So better record
+  // this information here, in case we later OSR back into this loop or
+  // reoptimize the whole function w/o rerunning the loop with the slow
+  // mode object in fullcodegen (which would result in a deopt loop).
+  __ EmitLoadTypeFeedbackVector(rdx);
+  __ Move(FieldOperand(rdx, FixedArray::OffsetOfElementAt(vector_index)),
+          TypeFeedbackVector::MegamorphicSentinel(isolate()));
+
   // Convert the entry to a string or null if it isn't a property
   // anymore. If the property has been removed while iterating, we
   // just skip it.