DFG CompareEq(a, null) and CompareStrictEq(a, const) are unsound with respect to constant folding
https://bugs.webkit.org/show_bug.cgi?id=109387

Reviewed by Oliver Hunt.
        
Lock in the decision to use a non-speculative constant comparison as early as possible
and don't let the CFA change it by folding constants. This might be a performance
penalty on some really weird code (FWIW, I haven't seen this on benchmarks), but on
the other hand it completely side-steps the unsoundness that the bug speaks of.

* dfg/DFGAbstractState.cpp:
(JSC::DFG::AbstractState::execute):
* dfg/DFGByteCodeParser.cpp:
(JSC::DFG::ByteCodeParser::isConstantForCompareStrictEq):
(ByteCodeParser):
(JSC::DFG::ByteCodeParser::parseBlock):
* dfg/DFGCSEPhase.cpp:
(JSC::DFG::CSEPhase::performNodeCSE):
* dfg/DFGNodeType.h:
(DFG):
* dfg/DFGPredictionPropagationPhase.cpp:
(JSC::DFG::PredictionPropagationPhase::propagate):
* dfg/DFGSpeculativeJIT.cpp:
(JSC::DFG::SpeculativeJIT::compileStrictEq):
* dfg/DFGSpeculativeJIT64.cpp:
(JSC::DFG::SpeculativeJIT::compile):



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@142491 268f45cc-cd09-0410-ab3c-d52691b4dbfc