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

src/full-codegen/arm64/full-codegen-arm64.cc

Index: src/full-codegen/arm64/full-codegen-arm64.cc
diff --git a/src/full-codegen/arm64/full-codegen-arm64.cc b/src/full-codegen/arm64/full-codegen-arm64.cc
index c743aa0f0545db436a55dafeaa3d83fa7486519e..2c57cf4017ac799da7532126e190dc079d037a98 100644
--- a/src/full-codegen/arm64/full-codegen-arm64.cc
+++ b/src/full-codegen/arm64/full-codegen-arm64.cc
@@ -1114,9 +1114,9 @@ void FullCodeGenerator::VisitForInStatement(ForInStatement* stmt) {
   // We got a fixed array in register x0. Iterate through that.
   __ Bind(&fixed_array);
 
+  int const vector_index = SmiFromSlot(slot)->value();
   __ EmitLoadTypeFeedbackVector(x1);
   __ Mov(x10, Operand(TypeFeedbackVector::MegamorphicSentinel(isolate())));
-  int vector_index = SmiFromSlot(slot)->value();
   __ Str(x10, FieldMemOperand(x1, FixedArray::OffsetOfElementAt(vector_index)));
   __ Mov(x1, Smi::FromInt(1));  // Smi(1) indicates slow check.
   __ Ldr(x2, FieldMemOperand(x0, FixedArray::kLengthOffset));
@@ -1150,6 +1150,16 @@ void FullCodeGenerator::VisitForInStatement(ForInStatement* stmt) {
   __ Cmp(x11, x2);
   __ 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(x0);
+  __ Mov(x10, Operand(TypeFeedbackVector::MegamorphicSentinel(isolate())));
+  __ Str(x10, FieldMemOperand(x0, 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.