fourthTier: each DFG node that relies on other nodes to do their type checks should be able to tell you if those type checks happened
https://bugs.webkit.org/show_bug.cgi?id=118866

Reviewed by Sam Weinig.

Adds a safeToExecute() method that takes a node and an abstract state and tells you
if the node will run without crashing under that state.

* JavaScriptCore.xcodeproj/project.pbxproj:
* bytecode/CodeBlock.cpp:
(JSC::CodeBlock::CodeBlock):
* dfg/DFGCFAPhase.cpp:
(CFAPhase):
(JSC::DFG::CFAPhase::CFAPhase):
(JSC::DFG::CFAPhase::run):
(JSC::DFG::CFAPhase::performBlockCFA):
(JSC::DFG::CFAPhase::performForwardCFA):
* dfg/DFGSafeToExecute.h: Added.
(DFG):
(SafeToExecuteEdge):
(JSC::DFG::SafeToExecuteEdge::SafeToExecuteEdge):
(JSC::DFG::SafeToExecuteEdge::operator()):
(JSC::DFG::SafeToExecuteEdge::result):
(JSC::DFG::safeToExecute):
* dfg/DFGStructureAbstractValue.h:
(JSC::DFG::StructureAbstractValue::isValidOffset):
(StructureAbstractValue):
* runtime/Options.h:
(JSC):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@153290 268f45cc-cd09-0410-ab3c-d52691b4dbfc