Indexing should only be computed when the new structure has an indexing header.

Indexing should only be computed when the new structure has an indexing header.
https://bugs.webkit.org/show_bug.cgi?id=180895

Reviewed by Saam Barati.

If we don't have an indexing header then we point the butterfly
sizeof(IndexingHeader) past the end of the butterfly. This makes
the computation of the offset simpler since it doesn't depend on
the indexing headeriness of the butterfly.

* jit/JITOperations.cpp:
* runtime/JSObject.cpp:
(JSC::JSObject::createInitialUndecided):
(JSC::JSObject::createInitialInt32):
(JSC::JSObject::createInitialDouble):
(JSC::JSObject::createInitialContiguous):
(JSC::JSObject::createArrayStorage):
(JSC::JSObject::convertUndecidedToArrayStorage):
(JSC::JSObject::convertInt32ToArrayStorage):
(JSC::JSObject::convertDoubleToArrayStorage):
* runtime/JSObject.h:
(JSC::JSObject::setButterfly):
(JSC::JSObject::nukeStructureAndSetButterfly):
* runtime/JSObjectInlines.h:
(JSC::JSObject::prepareToPutDirectWithoutTransition):
(JSC::JSObject::putDirectInternal):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@226000 268f45cc-cd09-0410-ab3c-d52691b4dbfc