Don't use kScratchRegister in DeserializeLazy

kScratchRegister is not guaranteed to be preserved across calls to
RecordWriteField.

Bug: v8:6844
Change-Id: I65377852807f52be821be4d5911b07886102c5ee
Reviewed-on: https://chromium-review.googlesource.com/709114Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48427}

src/builtins/x64/builtins-x64.cc

@@ -1384,7 +1384,7 @@ void Builtins::Generate_DeserializeLazy(MacroAssembler* masm) {
 
   Register target = rdi;  // Must be preserved
   Register scratch0 = rbx;
-  Register scratch1 = kScratchRegister;
+  Register scratch1 = r12;
 
   CHECK(scratch0 != rax && scratch0 != rdx && scratch0 != rdi);
   CHECK(scratch1 != rax && scratch1 != rdx && scratch1 != rdi);