https://bugs.webkit.org/show_bug.cgi?id=118778

Reviewed by Oliver Hunt.

Add a loop pre-header creation phase. Any loop that doesn't already have
just one predecessor that isn't part of the loop has a pre-header
prepended. All non-loop predecessors then jump to that pre-header.

Also fix a handful of bugs:

- DFG::Analysis should set m_valid before running the analysis, since that
  makes it easier to use ASSERT(m_valid) in the analysis' methods, which
  may be called by the analysis before the analysis completes. NaturalLoops
  does this with loopsOf().

- NaturalLoops::headerOf() was missing a check for innerMostLoopOf()
  returning 0, since that'll happen if the block isn't in any loop.

- Change BlockInsertionSet to dethread the graph, since anyone using it
  will want to do so.

- Change dethreading to ignore SSA form graphs.

This also adds NaturalLoops::belongsTo(), which I always used in the
pre-header creation phase. I didn't end up using it but I'll probably use
it in the near future.

* JavaScriptCore.xcodeproj/project.pbxproj:
* dfg/DFGAnalysis.h:
(JSC::DFG::Analysis::computeIfNecessary):
* dfg/DFGBlockInsertionSet.cpp:
(JSC::DFG::BlockInsertionSet::execute):
* dfg/DFGCriticalEdgeBreakingPhase.cpp:
(JSC::DFG::CriticalEdgeBreakingPhase::breakCriticalEdge):
* dfg/DFGGraph.cpp:
(JSC::DFG::Graph::dethread):
* dfg/DFGLoopPreHeaderCreationPhase.cpp: Added.
(DFG):
(LoopPreHeaderCreationPhase):
(JSC::DFG::LoopPreHeaderCreationPhase::LoopPreHeaderCreationPhase):
(JSC::DFG::LoopPreHeaderCreationPhase::run):
(JSC::DFG::performLoopPreHeaderCreation):
* dfg/DFGLoopPreHeaderCreationPhase.h: Added.
(DFG):
* dfg/DFGNaturalLoops.h:
(NaturalLoop):
(JSC::DFG::NaturalLoops::headerOf):
(JSC::DFG::NaturalLoops::innerMostLoopOf):
(JSC::DFG::NaturalLoops::innerMostOuterLoop):
(JSC::DFG::NaturalLoops::belongsTo):
(NaturalLoops):
* dfg/DFGPlan.cpp:
(JSC::DFG::Plan::compileInThreadImpl):

Conflicts:
	Source/JavaScriptCore/JavaScriptCore.xcodeproj/project.pbxproj

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@153279 268f45cc-cd09-0410-ab3c-d52691b4dbfc