Add some comments and remove dead while-loop.

Thanks @atrick for reviewing!

Author: eeckstein

include/swift/SILOptimizer/Analysis/BasicCalleeAnalysis.h

@@ -138,14 +138,23 @@ class BasicCalleeAnalysis : public SILAnalysis {
   }
 
   /// Invalidate all of the information for a specific function.
-  virtual void invalidate(SILFunction *F, InvalidationKind K) override { }
+  virtual void invalidate(SILFunction *F, InvalidationKind K) override {
+    // No invalidation needed because the analysis does not cache anything
+    // per call-site in functions.
+  }
 
   /// Notify the analysis about a newly created function.
-  virtual void notifyAddFunction(SILFunction *F) override { }
+  virtual void notifyAddFunction(SILFunction *F) override {
+    // Nothing to be done because the analysis does not cache anything
+    // per call-site in functions.
+  }
 
   /// Notify the analysis about a function which will be deleted from the
   /// module.
-  virtual void notifyDeleteFunction(SILFunction *F) override { };
+  virtual void notifyDeleteFunction(SILFunction *F) override {
+    // No invalidation needed because the analysis does not cache anything
+    // per call-site in functions.
+  };
 
   /// Notify the analysis about changed witness or vtables.
   virtual void invalidateFunctionTables() override {

include/swift/SILOptimizer/Analysis/DestructorAnalysis.h

@@ -35,10 +35,16 @@ class DestructorAnalysis : public SILAnalysis {
   bool mayStoreToMemoryOnDestruction(SILType T);
 
   /// No invalidation is needed.
-  virtual void invalidate() override { }
+  virtual void invalidate() override {
+    // Nothing can invalidate, because types are static and cannot be changed
+    // during the SIL pass pipeline.
+  }
 
   /// No invalidation is needed.
-  virtual void invalidate(SILFunction *F, InvalidationKind K)  override { }
+  virtual void invalidate(SILFunction *F, InvalidationKind K)  override {
+    // Nothing can invalidate, because types are static and cannot be changed
+    // during the SIL pass pipeline.
+  }
 
   /// Notify the analysis about a newly created function.
   virtual void notifyAddFunction(SILFunction *F) override { }

lib/SILOptimizer/IPO/CapturePromotion.cpp

@@ -970,9 +970,6 @@ class CapturePromotionPass : public SILModuleTransform {
     for (auto &F : *getModule())
       processFunction(&F, Worklist);
 
-      if (!Worklist.empty()) {
-      }
-
     while (!Worklist.empty())
       processFunction(Worklist.pop_back_val(), Worklist);
   }