RegExp matches arrays should use contiguous indexing.

RegExp matches arrays should use contiguous indexing.
<https://webkit.org/b/144286>

Reviewed by Geoffrey Garen.

We had a custom Structure being used for RegExp matches arrays that would
put the arrays into SlowPutArrayStorageShape mode. This was just left
from when matches arrays were custom, lazily initialized objects.

This change removes that Structure and switches the matches arrays to
using the default ContiguousShape Structure. This allows the FTL JIT
to compile the inner loop of the Octane/regexp benchmark.

Also made a version of initializeIndex() [inline] that takes the indexing
type in an argument, allowing createRegExpMatchesArray() to initialize
the entire array without branching on the indexing type for each entry.

~3% progression on Octane/regexp.

* runtime/JSGlobalObject.cpp:
(JSC::JSGlobalObject::init):
(JSC::JSGlobalObject::visitChildren):
* runtime/JSGlobalObject.h:
(JSC::JSGlobalObject::mapStructure):
(JSC::JSGlobalObject::regExpMatchesArrayStructure): Deleted.
* runtime/JSObject.h:
(JSC::JSObject::initializeIndex):
* runtime/RegExpMatchesArray.cpp:
(JSC::createRegExpMatchesArray):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@183438 268f45cc-cd09-0410-ab3c-d52691b4dbfc