[codegen] Swap using xchgq rather than three movq on x64

R=sigurds@chromium.org

Change-Id: Icd6e66b9bb23ca0dbf9fc5fe5c737a0bc3056a89
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1678359
Auto-Submit: Thibaud Michaud <thibaudm@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62386}

src/compiler/backend/x64/code-generator-x64.cc

@@ -4121,9 +4121,7 @@ void CodeGenerator::AssembleSwap(InstructionOperand* source,
       if (source->IsRegister()) {
         Register src = g.ToRegister(source);
         Register dst = g.ToRegister(destination);
-        __ movq(kScratchRegister, src);
-        __ movq(src, dst);
-        __ movq(dst, kScratchRegister);
+        __ xchgq(src, dst);
       } else {
         DCHECK(source->IsFPRegister());
         XMMRegister src = g.ToDoubleRegister(source);