SILPassManager: A new method to reduce the number of SIL pass runs.

It avoids that a pass runs a second time if didn't make any changes in the previous run and no other pass changed the function in between.
rdar://problem/20336764

In this change I also removed the CompleteFunctions analysis which is now obsolete.

Some measurements with swiftbench (-wmo, single threaded):
It reduces the number of pass runs by about 28%. Because only passes are skipped that don't do anything, the effect on compile time is not so dramatic.
The time spent in runSILOptimizationPasses is reduced by ~9% which gives a total compile time reduction of about 3%.




Swift SVN r26757

lib/SILAnalysis/Analysis.cpp

@@ -27,20 +27,6 @@
 
 using namespace swift;
 
-// anchor for virtual D'tor
-CompleteFunctions::~CompleteFunctions() {}
-
-void CompleteFunctions::setComplete() {
-  CompleteFuncs.clear();
-  if (!IsModulePending)
-    for (auto &F : *M)
-      if (!PendingFuncs.count(&F))
-        CompleteFuncs.insert(&F);
-
-  PendingFuncs.clear();
-  IsModulePending = false;
-}
-
 SILAnalysis *swift::createCallGraphAnalysis(SILModule *M) {
   return new CallGraphAnalysis(M);
 }