s390: fix compare and swap 64 loop

R=joransiu@ca.ibm.com

Change-Id: Ia33e925339c66fcefb9c3739c1985477190dab44
Reviewed-on: https://chromium-review.googlesource.com/c/1294372Reviewed-by: Joran Siu <joransiu@ca.ibm.com>
Commit-Queue: Junliang Yan <jyan@ca.ibm.com>
Cr-Commit-Position: refs/heads/master@{#56871}

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

@@ -2784,8 +2784,8 @@ CodeGenerator::CodeGenResult CodeGenerator::AssembleArchInstruction(
       Label do_cs;
       __ la(r1, MemOperand(base, index));
       __ lg(output, MemOperand(r1));
-      __ csg(output, value, MemOperand(r1));
       __ bind(&do_cs);
+      __ csg(output, value, MemOperand(r1));
       __ bne(&do_cs, Label::kNear);
       break;
     }