Remove write barriers for cells on x64, ARM and MIPS.

src/arm/lithium-codegen-arm.cc

Index: src/arm/lithium-codegen-arm.cc
===================================================================
--- src/arm/lithium-codegen-arm.cc	(revision 10185)
+++ src/arm/lithium-codegen-arm.cc	(working copy)
@@ -2283,21 +2283,7 @@
 
   // Store the value.
   __ str(value, FieldMemOperand(scratch, JSGlobalPropertyCell::kValueOffset));
-
-  // Cells are always in the remembered set.
-  if (instr->hydrogen()->NeedsWriteBarrier()) {
-    HType type = instr->hydrogen()->value()->type();
-    SmiCheck check_needed =
-        type.IsHeapObject() ? OMIT_SMI_CHECK : INLINE_SMI_CHECK;
-    __ RecordWriteField(scratch,
-                        JSGlobalPropertyCell::kValueOffset,
-                        value,
-                        scratch2,
-                        kLRHasBeenSaved,
-                        kSaveFPRegs,
-                        OMIT_REMEMBERED_SET,
-                        check_needed);
-  }
+  // Cells are always rescanned, so no write barrier here.
 }