DFG string concatenation optimizations might emit speculative nodes after emitting nodes that kill the original inputs
https://bugs.webkit.org/show_bug.cgi?id=119032

Reviewed by Oliver Hunt.

Source/JavaScriptCore: 

It just needs some Phantom action.

* dfg/DFGFixupPhase.cpp:
(JSC::DFG::FixupPhase::attemptToMakeFastStringAdd):

LayoutTests: 

* fast/js/dfg-strcat-over-objects-then-exit-on-it-expected.txt: Added.
* fast/js/dfg-strcat-over-objects-then-exit-on-it.html: Added.
* fast/js/script-tests/dfg-strcat-over-objects-then-exit-on-it.js: Added.
(foo):
(bar):
(x):



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@153075 268f45cc-cd09-0410-ab3c-d52691b4dbfc