s390: load zero on r0 instead of xgr

xgr kills condition code on the branch

R=joransiu@ca.ibm.com

Change-Id: I90a75760c96319d8f27512395f904796a114c5e9
Reviewed-on: https://chromium-review.googlesource.com/950135Reviewed-by: Joran Siu <joransiu@ca.ibm.com>
Commit-Queue: Junliang Yan <jyan@ca.ibm.com>
Cr-Commit-Position: refs/heads/master@{#51748}

src/compiler/s390/code-generator-s390.cc

@@ -2459,7 +2459,7 @@ void CodeGenerator::AssembleBranchPoisoning(FlagsCondition condition,
   }
 
   condition = NegateFlagsCondition(condition);
-  __ XorP(r0, r0);
+  __ LoadImmP(r0, Operand::Zero());
   __ LoadOnConditionP(FlagsConditionToCondition(condition, kArchNop),
                       kSpeculationPoisonRegister, r0);
 }