Add mistagging-readbarrier to weak cell

src/ia32/lithium-ia32.cc

Index: src/ia32/lithium-ia32.cc
diff --git a/src/ia32/lithium-ia32.cc b/src/ia32/lithium-ia32.cc
index 51a9b62fb25b1fda34be603146569fc310595329..811d1561816a2d8e72ba44c48acc780e6a07f9ac 100644
--- a/src/ia32/lithium-ia32.cc
+++ b/src/ia32/lithium-ia32.cc
@@ -2171,10 +2171,17 @@ LInstruction* LChunkBuilder::DoStoreContextSlot(HStoreContextSlot* instr) {
 
 
 LInstruction* LChunkBuilder::DoLoadNamedField(HLoadNamedField* instr) {
-  LOperand* obj = (instr->access().IsExternalMemory() &&
-                   instr->access().offset() == 0)
-      ? UseRegisterOrConstantAtStart(instr->object())
-      : UseRegisterAtStart(instr->object());
+  LOperand* obj;
+  if (instr->access().read_barrier()) {
+    DCHECK(!instr->access().IsExternalMemory());
+    obj = UseRegister(instr->object());
+  } else {
+    // With no read barrier we can have the same register for the object and
+    // the field.
+    obj = (instr->access().IsExternalMemory() && instr->access().offset() == 0)
+              ? UseRegisterOrConstantAtStart(instr->object())
+              : UseRegisterAtStart(instr->object());
+  }
   return DefineAsRegister(new(zone()) LLoadNamedField(obj));
 }