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

src/full-codegen/mips64/full-codegen-mips64.cc

Index: src/full-codegen/mips64/full-codegen-mips64.cc
diff --git a/src/full-codegen/mips64/full-codegen-mips64.cc b/src/full-codegen/mips64/full-codegen-mips64.cc
index 9f32a88ce72170e75ad5eec05729e81bc38bc84a..aa36b98fb913c02d4edea1aab9d9d447ec9557f0 100644
--- a/src/full-codegen/mips64/full-codegen-mips64.cc
+++ b/src/full-codegen/mips64/full-codegen-mips64.cc
@@ -1114,9 +1114,9 @@ void FullCodeGenerator::VisitForInStatement(ForInStatement* stmt) {
   // We got a fixed array in register v0. Iterate through that.
   __ bind(&fixed_array);
 
+  int const vector_index = SmiFromSlot(slot)->value();
   __ EmitLoadTypeFeedbackVector(a1);
   __ li(a2, Operand(TypeFeedbackVector::MegamorphicSentinel(isolate())));
-  int vector_index = SmiFromSlot(slot)->value();
   __ sd(a2, FieldMemOperand(a1, FixedArray::OffsetOfElementAt(vector_index)));
 
   __ li(a1, Operand(Smi::FromInt(1)));  // Smi(1) indicates slow check
@@ -1154,6 +1154,16 @@ void FullCodeGenerator::VisitForInStatement(ForInStatement* stmt) {
   __ ld(a4, FieldMemOperand(a1, HeapObject::kMapOffset));
   __ Branch(&update_each, eq, a4, Operand(a2));
 
+  // 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(a0);
+  __ li(a2, Operand(TypeFeedbackVector::MegamorphicSentinel(isolate())));
+  __ sd(a2, FieldMemOperand(a0, 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.