[Liftoff] Don't allocate handle for self-reference

Liftoff does not need to reference its own code object, so don't
allocate a handle for that purpose.
This also allows to create LiftoffAssembler instances in background
tasks where no handle scope is available.

R=ahaas@chromium.org

Bug: v8:6600
Change-Id: I2911aebd14aaa8450456cb8eea16cea547e0b671
Reviewed-on: https://chromium-review.googlesource.com/972081Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52098}

src/wasm/baseline/liftoff-assembler.cc

@@ -338,7 +338,7 @@ void LiftoffAssembler::CacheState::Split(const CacheState& source) {
 // TODO(clemensh): Provide a reasonably sized buffer, based on wasm function
 // size.
 LiftoffAssembler::LiftoffAssembler(Isolate* isolate)
-    : TurboAssembler(isolate, nullptr, 0, CodeObjectRequired::kYes) {}
+    : TurboAssembler(isolate, nullptr, 0, CodeObjectRequired::kNo) {}
 
 LiftoffAssembler::~LiftoffAssembler() {
   if (num_locals_ > kInlineLocalTypes) {