[JSC] Handle PhantomSpread in LoadVarargs as the same to the others

[JSC] Handle PhantomSpread in LoadVarargs as the same to the others
https://bugs.webkit.org/show_bug.cgi?id=171262

Reviewed by Saam Barati.

JSTests:

* stress/spread-outer-create-rest.js: Added.
(assert):
(foo):
(bar):
(baz):

Source/JavaScriptCore:

This is follow-up patch after r215720. In that patch, accidentally
we did not apply the same change to LoadVarargs in argument elimination
phase. This patch just does the same rewriting to handle PhantomSpread
correctly.

* dfg/DFGArgumentsEliminationPhase.cpp:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@215860 268f45cc-cd09-0410-ab3c-d52691b4dbfc