PPC: Cross-script variables handling fixed. It was possible to write to read-only global variable.

Port 156042f7

R=ishell@chromium.org, dstence@us.ibm.com, michael_dawson@ca.ibm.com
BUG=

Review URL: https://codereview.chromium.org/1259583003

Cr-Commit-Position: refs/heads/master@{#29880}

src/ppc/code-stubs-ppc.cc

@@ -5377,6 +5377,11 @@ void StoreGlobalViaContextStub::Generate(MacroAssembler* masm) {
   __ add(cell, context, r0);
   __ LoadP(cell, ContextOperand(cell));
 
+  // Check that cell value is not the_hole.
+  __ LoadP(cell_value, FieldMemOperand(cell, PropertyCell::kValueOffset));
+  __ CompareRoot(cell_value, Heap::kTheHoleValueRootIndex);
+  __ beq(&slow_case);
+
   // Load PropertyDetails for the cell (actually only the cell_type and kind).
   __ LoadP(cell_details, FieldMemOperand(cell, PropertyCell::kDetailsOffset));
   __ SmiUntag(cell_details);
@@ -5406,7 +5411,6 @@ void StoreGlobalViaContextStub::Generate(MacroAssembler* masm) {
   // Check if PropertyCell value matches the new value (relevant for Constant,
   // ConstantType and Undefined cells).
   Label not_same_value;
-  __ LoadP(cell_value, FieldMemOperand(cell, PropertyCell::kValueOffset));
   __ cmp(cell_value, value);
   __ bne(&not_same_value);