Unify the data access of StringImpl members from JavaScriptCore
https://bugs.webkit.org/show_bug.cgi?id=115320

Reviewed by Andreas Kling.

DFG accesses the member infos by directly calling the methods on StringImpl,
while the baseline JIT was using helper methods on ThunkHelpers.

Cut the middle man, and use StringImpl directly everywhere.

* jit/JITInlines.h:
(JSC::JIT::emitLoadCharacterString):
* jit/JITPropertyAccess.cpp:
(JSC::JIT::stringGetByValStubGenerator):
* jit/JITPropertyAccess32_64.cpp:
(JSC::JIT::stringGetByValStubGenerator):
* jit/JSInterfaceJIT.h:
* jit/ThunkGenerators.cpp:
(JSC::stringCharLoad):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@149344 268f45cc-cd09-0410-ab3c-d52691b4dbfc