DFG doesn't support to_jsnumber

DFG doesn't support to_jsnumber
https://bugs.webkit.org/show_bug.cgi?id=115129

Reviewed by Geoffrey Garen.

Based on Oliver's patch. Implements to_jsnumber as Identity(Number:@thingy), and then does
an optimization in Fixup to turn Identity(Number:) into Identity(Int32:) if the predictions
tell us to. Identity is later turned into Phantom.

Also fixed BackPropMask, which appeared to have NodeDoesNotExit included in it. That's
wrong; NodeDoesNotExit is not a backward propagation property.

Also fixed Identity to be marked as CanExit (i.e. not NodeDoesNotExit).

This more than doubles the FPS on ammo.

* dfg/DFGByteCodeParser.cpp:
(JSC::DFG::ByteCodeParser::parseBlock):
* dfg/DFGCapabilities.h:
(JSC::DFG::canCompileOpcode):
* dfg/DFGFixupPhase.cpp:
(JSC::DFG::FixupPhase::fixupNode):
(FixupPhase):
(JSC::DFG::FixupPhase::observeUseKindOnNode):
(JSC::DFG::FixupPhase::observeUseKindOnEdge):
* dfg/DFGNodeFlags.h:
(DFG):
* dfg/DFGNodeType.h:
(DFG):
* dfg/DFGPredictionPropagationPhase.cpp:
(JSC::DFG::PredictionPropagationPhase::propagate):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@149162 268f45cc-cd09-0410-ab3c-d52691b4dbfc