Remove redundant logic in AllowedToBeMigrated

Simplifies boolean logic since shortcuts mean most will never be
executed and also replaces variables with constants to improve
readability.

Change-Id: Ic1678bc60c3e85daced758ce139f7d03c25632dc
Reviewed-on: https://chromium-review.googlesource.com/928320Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51419}

src/heap/heap.cc

@@ -6644,12 +6644,11 @@ bool Heap::AllowedToBeMigrated(HeapObject* obj, AllocationSpace dst) {
   AllocationSpace src = chunk->owner()->identity();
   switch (src) {
     case NEW_SPACE:
-      return dst == src || dst == OLD_SPACE;
+      return dst == NEW_SPACE || dst == OLD_SPACE;
     case OLD_SPACE:
-      return dst == src &&
-             (dst == OLD_SPACE || obj->IsFiller() || obj->IsExternalString());
+      return dst == OLD_SPACE;
     case CODE_SPACE:
-      return dst == src && type == CODE_TYPE;
+      return dst == CODE_SPACE && type == CODE_TYPE;
     case MAP_SPACE:
     case LO_SPACE:
       return false;