PPC: fix debug build break

Fix debug build break by using pc-relative code on ComputeCodeStartAddress

R=joransiu@ca.ibm.com

Change-Id: I81c3a3e220c9f4f90772bcb39f1a9b9d48661e19
Reviewed-on: https://chromium-review.googlesource.com/1222606Reviewed-by: Joran Siu <joransiu@ca.ibm.com>
Commit-Queue: Junliang Yan <jyan@ca.ibm.com>
Cr-Commit-Position: refs/heads/master@{#55840}

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

@@ -882,9 +882,7 @@ void CodeGenerator::BailoutIfDeoptimized() {
 void CodeGenerator::GenerateSpeculationPoisonFromCodeStartRegister() {
   Register scratch = kScratchReg;
 
-  __ mflr(r0);
-  __ LoadPC(scratch);
-  __ subi(scratch, scratch, Operand(__ pc_offset() - kInstrSize));
+  __ ComputeCodeStartAddress(scratch);
 
   // Calculate a mask which has all bits set in the normal case, but has all
   // bits cleared if we are speculatively executing the wrong PC.
@@ -893,7 +891,6 @@ void CodeGenerator::GenerateSpeculationPoisonFromCodeStartRegister() {
   __ notx(kSpeculationPoisonRegister, scratch);
   __ isel(eq, kSpeculationPoisonRegister,
           kSpeculationPoisonRegister, scratch);
-  __ mtlr(r0);
 }
 
 void CodeGenerator::AssembleRegisterArgumentPoisoning() {

src/ppc/macro-assembler-ppc.cc

@@ -906,11 +906,10 @@ void TurboAssembler::LoadPC(Register dst) {
 }
 
 void TurboAssembler::ComputeCodeStartAddress(Register dst) {
-  Label current_pc;
-  mov_label_addr(dst, &current_pc);
-
-  bind(&current_pc);
-  subi(dst, dst, Operand(pc_offset()));
+  mflr(r0);
+  LoadPC(dst);
+  subi(dst, dst, Operand(pc_offset() - kInstrSize));
+  mtlr(r0);
 }
 
 void TurboAssembler::LoadConstantPoolPointerRegister() {