PPC: Set const pool unavailable after frame destructed

R=joransiu@ca.ibm.com, jbarboza@ca.ibm.com, michael_dawson@ca.ibm.com

Bug: 
Change-Id: I1f25a81637dd50b1d7e9a47154e3df4c61521fad
Reviewed-on: https://chromium-review.googlesource.com/744504Reviewed-by: Joran Siu <joransiu@ca.ibm.com>
Commit-Queue: Junliang Yan <jyan@ca.ibm.com>
Cr-Commit-Position: refs/heads/master@{#49043}

src/compiler/ppc/code-generator-ppc.cc

@@ -2425,6 +2425,8 @@ void CodeGenerator::AssembleReturn(InstructionOperand* pop) {
       AssembleDeconstructFrame();
     }
   }
+  // Constant pool is unavailable since the frame has been destructed
+  ConstantPoolUnavailableScope constant_pool_unavailable(tasm());
   if (pop->IsImmediate()) {
     DCHECK_EQ(Constant::kInt32, g.ToConstant(pop).type());
     pop_count += g.ToConstant(pop).ToInt32();