Merge pull request swiftlang#15961 from slavapestov/re-run-predictable-mem-opts

SIL Optimizer: Re-run predictable memory optimizations in performance pipeline

Author: slavapestov

lib/SILOptimizer/PassManager/PassPipeline.cpp

@@ -223,6 +223,10 @@ void addSSAPasses(SILPassPipelinePlan &P, OptimizationLevelKind OpLevel) {
   // Split up operations on stack-allocated aggregates (struct, tuple).
   P.addSROA();
 
+  // Re-run predictable memory optimizations, since previous optimization
+  // passes sometimes expose oppotunities here.
+  P.addPredictableMemoryOptimizations();
+
   // Promote stack allocations to values.
   P.addMem2Reg();