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

src/full-codegen/mips/full-codegen-mips.cc

Index: src/full-codegen/mips/full-codegen-mips.cc
diff --git a/src/full-codegen/mips/full-codegen-mips.cc b/src/full-codegen/mips/full-codegen-mips.cc
index 67e36b8dba34bae2cb18b9d09263af7a5feedeeb..af1b2304ef4a889ef632c0cc9822cdded37ae25e 100644
--- a/src/full-codegen/mips/full-codegen-mips.cc
+++ b/src/full-codegen/mips/full-codegen-mips.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();
   __ sw(a2, FieldMemOperand(a1, FixedArray::OffsetOfElementAt(vector_index)));
 
   __ li(a1, Operand(Smi::FromInt(1)));  // Smi(1) indicates slow check
@@ -1153,6 +1153,16 @@ void FullCodeGenerator::VisitForInStatement(ForInStatement* stmt) {
   __ lw(t0, FieldMemOperand(a1, HeapObject::kMapOffset));
   __ Branch(&update_each, eq, t0, 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())));
+  __ sw(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.