[compiler] Change implementation of IsMaterializableFromRoot.

RootCanBeTreatedAsConstant checks that the object is not in new space.
This check is not thread-safe and, moreover, makes no sense in Turbofan.

R=bmeurer@chromium.org

Bug: v8:6048
Change-Id: I6df682ceef1508c33d5f32bd0d5bf25828e8c88f
Reviewed-on: https://chromium-review.googlesource.com/565259
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46509}

src/compiler/code-generator.cc

@@ -306,7 +306,7 @@ bool CodeGenerator::IsMaterializableFromRoot(
   if (incoming_descriptor->flags() & CallDescriptor::kCanUseRoots) {
     Heap* heap = isolate()->heap();
     return heap->IsRootHandle(object, index_return) &&
-           heap->RootCanBeTreatedAsConstant(*index_return);
+           !heap->RootCanBeWrittenAfterInitialization(*index_return);
   }
   return false;
 }