Implement get_scoped_var in the DFG

Implement get_scoped_var in the DFG
https://bugs.webkit.org/show_bug.cgi?id=68640

Reviewed by Gavin Barraclough.

Naive implementation of get_scoped_var in the DFG.  Essentially this
is the bare minimum required to get correct behaviour, so there's no
load/store coalescing or type profiling involved, even though these
would be wins.  No impact on SunSpider or V8.

* dfg/DFGByteCodeParser.cpp:
(JSC::DFG::ByteCodeParser::parseBlock):
* dfg/DFGCapabilities.h:
(JSC::DFG::canCompileOpcode):
* dfg/DFGNode.h:
(JSC::DFG::Node::hasVarNumber):
(JSC::DFG::Node::hasScopeChainDepth):
(JSC::DFG::Node::scopeChainDepth):
* dfg/DFGPropagator.cpp:
(JSC::DFG::Propagator::propagateNodePredictions):
* dfg/DFGSpeculativeJIT.cpp:
(JSC::DFG::SpeculativeJIT::compile):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@95742 268f45cc-cd09-0410-ab3c-d52691b4dbfc