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

src/full-codegen/arm/full-codegen-arm.cc

Index: src/full-codegen/arm/full-codegen-arm.cc
diff --git a/src/full-codegen/arm/full-codegen-arm.cc b/src/full-codegen/arm/full-codegen-arm.cc
index a6a681bdfae05f34d5e415d7cef7aa20f0f21aa8..4208afbf2debfd80659d7627176888cd00bb19a8 100644
--- a/src/full-codegen/arm/full-codegen-arm.cc
+++ b/src/full-codegen/arm/full-codegen-arm.cc
@@ -1118,9 +1118,9 @@ void FullCodeGenerator::VisitForInStatement(ForInStatement* stmt) {
   // We got a fixed array in register r0. Iterate through that.
   __ bind(&fixed_array);
 
+  int const vector_index = SmiFromSlot(slot)->value();
   __ EmitLoadTypeFeedbackVector(r1);
   __ mov(r2, Operand(TypeFeedbackVector::MegamorphicSentinel(isolate())));
-  int vector_index = SmiFromSlot(slot)->value();
   __ str(r2, FieldMemOperand(r1, FixedArray::OffsetOfElementAt(vector_index)));
   __ mov(r1, Operand(Smi::FromInt(1)));  // Smi(1) indicates slow check
   __ Push(r1, r0);  // Smi and array
@@ -1156,6 +1156,16 @@ void FullCodeGenerator::VisitForInStatement(ForInStatement* stmt) {
   __ cmp(r4, Operand(r2));
   __ b(eq, &update_each);
 
+  // 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(r0);
+  __ mov(r2, Operand(TypeFeedbackVector::MegamorphicSentinel(isolate())));
+  __ str(r2, FieldMemOperand(r0, FixedArray::OffsetOfElementAt(vector_index)));
+
   // Convert the entry to a string or (smi) 0 if it isn't a property
   // any more. If the property has been removed while iterating, we
   // just skip it.