Remove 4/5 runs of dead store elimination. I did not measure a real performance difference on my local machine.
Remove 4/5 runs of dead store elimination. I did not measure a real performance difference on
my local machine.

rdar://24392141

This is going to cut compilation time spent in dead store elim by 5X

The last iteration of dead store ran just before the last iteration of arc-sequence-opt
allows us to catch some opportunites passes like Mem2Reg can not eliminate. And this allows
more code motion freedom.

Stdlib -O after removing 4/5 dead stores.
=========================================

Running Time	Self (ms)		Symbol Name
22082.0ms   37.1%	0.0	 	    swift::runSILOptimizationPasses(swift::SILModule&)
21905.0ms   36.8%	0.0	 	     swift::SILPassManager::runOneIteration()
17616.0ms   29.6%	35.0	 	      swift::SILPassManager::runFunctionPasses(llvm::ArrayRef<swift::SILFunctionTransform*>)
16667.0ms   28.0%	55.0	 	       swift::SILPassManager::runPassesOnFunction(llvm::ArrayRef<swift::SILFunctionTransform*>, swift::SILFunction*)
3063.0ms    5.1%	7.0	 	        (anonymous namespace)::SimplifyCFGPass::run()
2936.0ms    4.9%	20.0	 	        (anonymous namespace)::ARCSequenceOpts::run()
2343.0ms    3.9%	3.0	 	        (anonymous namespace)::SILCombine::run()
1900.0ms    3.1%	110.0	 	        (anonymous namespace)::SILCSE::run()
1642.0ms    2.7%	43.0	 	        (anonymous namespace)::RedundantLoadElimination::run()
1113.0ms    1.8%	6.0	 	        (anonymous namespace)::GenericSpecializer::run()
788.0ms    1.3%	120.0	 	        (anonymous namespace)::DCE::run()
495.0ms    0.8%	3.0	 	        (anonymous namespace)::SILCodeMotion::run()
304.0ms    0.5%	1.0	 	        (anonymous namespace)::StackPromotion::run()
292.0ms    0.4%	1.0	 	        (anonymous namespace)::ConstantPropagation::run()
269.0ms    0.4%	5.0	 	        (anonymous namespace)::ABCOpt::run()
236.0ms    0.3%	35.0	 	        (anonymous namespace)::SILSROA::run()
192.0ms    0.3%	2.0	 	        (anonymous namespace)::SILMem2Reg::run()
146.0ms    0.2%	65.0	 	        (anonymous namespace)::SILLowerAggregate::run()
132.0ms    0.2%	5.0	 	        (anonymous namespace)::LICM::run()
132.0ms    0.2%	7.0	 	        (anonymous namespace)::DeadStoreElimination::run()
96.0ms    0.1%	65.0	 	        (anonymous namespace)::Devirtualizer::run()
67.0ms    0.1%	59.0	 	        (anonymous namespace)::DeadObjectElimination::run()
62.0ms    0.1%	44.0	 	        (anonymous namespace)::RemovePinInsts::run()

StdlibUnitTest -O after removing 4/5 dead stores.
=================================================

Running Time	Self (ms)		Symbol Name
6958.0ms   26.9%	0.0	 	    swift::runSILOptimizationPasses(swift::SILModule&)
6923.0ms   26.8%	0.0	 	     swift::SILPassManager::runOneIteration()
5638.0ms   21.8%	5.0	 	      swift::SILPassManager::runFunctionPasses(llvm::ArrayRef<swift::SILFunctionTransform*>)
5363.0ms   20.7%	8.0	 	       swift::SILPassManager::runPassesOnFunction(llvm::ArrayRef<swift::SILFunctionTransform*>, swift::SILFunction*)
1535.0ms    5.9%	1.0	 	        (anonymous namespace)::ARCSequenceOpts::run()
789.0ms    3.0%	2.0	 	        (anonymous namespace)::SimplifyCFGPass::run()
704.0ms    2.7%	0.0	 	        (anonymous namespace)::SILCombine::run()
615.0ms    2.3%	36.0	 	        (anonymous namespace)::SILCSE::run()
506.0ms    1.9%	14.0	 	        (anonymous namespace)::RedundantLoadElimination::run()
224.0ms    0.8%	44.0	 	        (anonymous namespace)::DCE::run()
150.0ms    0.5%	1.0	 	        (anonymous namespace)::SILCodeMotion::run()
113.0ms    0.4%	1.0	 	        (anonymous namespace)::StackPromotion::run()
98.0ms    0.3%	4.0	 	        (anonymous namespace)::DeadStoreElimination::run()
80.0ms    0.3%	3.0	 	        (anonymous namespace)::ABCOpt::run()
74.0ms    0.2%	5.0	 	        (anonymous namespace)::LICM::run()

khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift