Move DFG::OSRExitCompiler methods into DFG::OSRExit [step 3].

Move DFG::OSRExitCompiler methods into DFG::OSRExit [step 3].
https://bugs.webkit.org/show_bug.cgi?id=175228
<rdar://problem/33735737>

Reviewed by Saam Barati.

Merge the 32-bit OSRExit::compileExit() method into the 64-bit version, and
delete OSRExit32_64.cpp.

* CMakeLists.txt:
* JavaScriptCore.xcodeproj/project.pbxproj:
* dfg/DFGOSRExit.cpp:
(JSC::DFG::OSRExit::compileExit):
* dfg/DFGOSRExit32_64.cpp: Removed.
* jit/GPRInfo.h:
(JSC::JSValueSource::payloadGPR const):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@220318 268f45cc-cd09-0410-ab3c-d52691b4dbfc