Move DCE to later in the SSA passes.

Doing it later doesn't get in the way of other optimizations, and could
result in removing more things that become dead in the meantime.

Author: rudkx

lib/SILOptimizer/PassManager/Passes.cpp

@@ -225,7 +225,6 @@ void AddSSAPasses(SILPassManager &PM, OptimizationLevelKind OpLevel) {
   AddSimplifyCFGSILCombine(PM);
 
   PM.addPerformanceConstantPropagation();
-  PM.addDCE();
   PM.addCSE();
   PM.addSILCombine();
   PM.addJumpThreadSimplifyCFG();
@@ -237,6 +236,7 @@ void AddSSAPasses(SILPassManager &PM, OptimizationLevelKind OpLevel) {
 
   // Perform retain/release code motion and run the first ARC optimizer.
   PM.addCSE();
+  PM.addDCE();
   PM.addEarlyCodeMotion();
   PM.addARCSequenceOpts();