Run more function passes in a single run of the pass manager.

This commit moves the SILLinker pass out of AddSSAPasses, so that we run
more function passes on each function before moving up to it's callers.

Now the only remaining module passes in AddSSAPasses are GlobalOpt and
LetPropertiesOpt, which run only when we call AddSSAPasses for the
MidLevel optimizations.

This commit also adds the high level loop opt passes onto the same pass
run. As a result of this and moving SILLinker out of AddSSAPasses, we
now run far more passes together on a given function before moving up
the call graph to the callers.

The net result is that I am now seeing approximately a 2% reduction in
stdlib compile times, with only a single significant performance
regression (there are some other minor improvements and regressions, and
some major improvements with -Ounchecked).

The 2% reduction appears to come largely from the mechanism in the pass
manager that skips running passes if we've not made any changes to a
function since the last time the pass was run.

Author: rudkx

lib/SILOptimizer/PassManager/Passes.cpp

@@ -239,8 +239,6 @@ void AddSSAPasses(SILPassManager &PM, OptimizationLevelKind OpLevel) {
   PM.addEarlyCodeMotion();
   PM.addARCSequenceOpts();
 
-  PM.addSILLinker();
-
   PM.addSimplifyCFG();
   // Only hoist releases very late.
   if (OpLevel == OptimizationLevelKind::LowLevel)
@@ -264,7 +262,7 @@ void swift::runSILOptimizationPasses(SILModule &Module) {
     return;
   }
 
-  SILPassManager PM(&Module, "PreSpecialize");
+  SILPassManager PM(&Module, "EarlyModulePasses");
 
   // Get rid of apparently dead functions as soon as possible so that
   // we do not spend time optimizing them.
@@ -275,15 +273,15 @@ void swift::runSILOptimizationPasses(SILModule &Module) {
   PM.resetAndRemoveTransformations();
 
   // Run an iteration of the high-level SSA passes.
-  PM.setStageName("HighLevel");
+  PM.setStageName("HighLevel+EarlyLoopOpt");
   AddSSAPasses(PM, OptimizationLevelKind::HighLevel);
+  AddHighLevelLoopOptPasses(PM);
   PM.runOneIteration();
   PM.resetAndRemoveTransformations();
 
-  PM.setStageName("EarlyLoopOpt");
-  AddHighLevelLoopOptPasses(PM);
-
+  PM.setStageName("MidModulePasses+StackPromote");
   PM.addDeadFunctionElimination();
+  PM.addSILLinker();
   PM.addDeadObjectElimination();
   PM.addGlobalPropertyOpt();