DFG constant folding and CFG simplification should be smart enough to know that if a logical op's operand is proven to have a non-masquerading structure then it always evaluates to true
https://bugs.webkit.org/show_bug.cgi?id=101511

Reviewed by Geoffrey Garen.
        
This is the second attempt at this patch, which fixes the !"" case.

To make life easier, this moves BranchDirection into BasicBlock so that after
running the CFA, we always know, for each block, what direction the CFA
proved. CFG simplification now both uses and preserves cfaBranchDirection in
its transformations.
        
Also made both LogicalNot and Branch check whether the operand is a known cell
with a known structure, and if so, made them do the appropriate folding.
        
5% speed-up on V8/raytrace because it makes raytrace's own null checks
evaporate (i.e. idioms like 'if (!x) throw "unhappiness"') thanks to the fact
that we were already doing structure check hoisting.

* JavaScriptCore.xcodeproj/project.pbxproj:
* dfg/DFGAbstractState.cpp:
(JSC::DFG::AbstractState::endBasicBlock):
(JSC::DFG::AbstractState::execute):
(JSC::DFG::AbstractState::mergeToSuccessors):
* dfg/DFGAbstractState.h:
(AbstractState):
* dfg/DFGBasicBlock.h:
(JSC::DFG::BasicBlock::BasicBlock):
(BasicBlock):
* dfg/DFGBranchDirection.h: Added.
(DFG):
(JSC::DFG::branchDirectionToString):
(JSC::DFG::isKnownDirection):
(JSC::DFG::branchCondition):
* dfg/DFGCFGSimplificationPhase.cpp:
(JSC::DFG::CFGSimplificationPhase::run):
(JSC::DFG::CFGSimplificationPhase::mergeBlocks):



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@134164 268f45cc-cd09-0410-ab3c-d52691b4dbfc