[turbofan] Eliminate a code dependency in Object.create lowering.

The dependency is unnecessary.

Also add a DCHECK that effectively ensures that the instance size can't
change.

NOPRESUBMIT=true

Bug: v8:7790
Change-Id: Ib68ed044f2f87a0b8b83f4110756f3d3ad96c530
Reviewed-on: https://chromium-review.googlesource.com/1110121
Commit-Queue: Georg Neis <neis@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53953}

src/compiler/js-create-lowering.cc

@@ -1492,7 +1492,7 @@ Reduction JSCreateLowering::ReduceJSCreateObject(Node* node) {
 
   int const instance_size = instance_map->instance_size();
   if (instance_size > kMaxRegularHeapObjectSize) return NoChange();
-  dependencies()->AssumeInitialMapCantChange(instance_map);
+  CHECK(!instance_map->IsInobjectSlackTrackingInProgress());
 
   // Emit code to allocate the JSObject instance for the given
   // {instance_map}.