Array.splice should be fast when it is used to remove elements other than the very first
https://bugs.webkit.org/show_bug.cgi?id=98236

Reviewed by Michael Saboff.

Applied the same technique that was used to optimize the unshift case of splice in
http://trac.webkit.org/changeset/129676.  This is a >20x speed-up on programs that
use splice for element removal.

* runtime/ArrayPrototype.cpp:
(JSC::shift):
* runtime/JSArray.cpp:
(JSC::JSArray::shiftCount):
* runtime/JSArray.h:
(JSArray):



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@130315 268f45cc-cd09-0410-ab3c-d52691b4dbfc