Air should eliminate dead code

Air should eliminate dead code
https://bugs.webkit.org/show_bug.cgi?id=150746

Reviewed by Geoffrey Garen.

This adds a very simple dead code elimination to Air. It simply looks at whether a Tmp or
StackSlot has ever been used by a live instruction. An instruction is live if it has non-arg
effects (branching, returning, calling, etc) or if it stores to a live Arg. An Arg is live if
it references a live Tmp or StackSlot, or if it is neither a Tmp nor a StackSlot. The phase
runs these rules to fixpoint, and then removes the dead instructions.

This also changes the AirOpcodes parser to handle multiple attributes per opcode, so that we
could conceivably say things like "FooBar /branch /effects". It also adds the /effects
attribute, which we currently use for Breakpoint and nothing else. C calls, patchpoints, and
checks are all Specials, and the Special base class by default always claims that the
instruction has effects. In the future, we could have B3 use a Patch in Air to implement
exotic math constructs; then the Special associated with that thing would claim that there
are no effects.

* JavaScriptCore.xcodeproj/project.pbxproj:
* b3/air/AirBasicBlock.h:
(JSC::B3::Air::BasicBlock::begin):
(JSC::B3::Air::BasicBlock::end):
(JSC::B3::Air::BasicBlock::at):
(JSC::B3::Air::BasicBlock::last):
(JSC::B3::Air::BasicBlock::resize):
(JSC::B3::Air::BasicBlock::appendInst):
* b3/air/AirEliminateDeadCode.cpp: Added.
(JSC::B3::Air::eliminateDeadCode):
* b3/air/AirEliminateDeadCode.h: Added.
* b3/air/AirGenerate.cpp:
(JSC::B3::Air::generate):
* b3/air/AirInst.h:
* b3/air/AirOpcode.opcodes:
* b3/air/AirSpecial.cpp:
(JSC::B3::Air::Special::name):
(JSC::B3::Air::Special::hasNonArgNonControlEffects):
(JSC::B3::Air::Special::dump):
* b3/air/AirSpecial.h:
* b3/air/opcode_generator.rb:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@191846 268f45cc-cd09-0410-ab3c-d52691b4dbfc