Remove two runs of the passes in AddSSAPasses.

I measure around a 10% reduction in compile times of release no-assert
builds of the stdlib and StdlibUnitTest.

For release + debug-swift builds, I see 20% reduction in stdlib compile
time.

I saw no reproducible regressions in the benchmarks, and a few
improvements.

There is a small (0.1%) reduction in the libswiftCore.dylib size.

Being able to remove these is a consequence of the reordering that
happened in e50daa6.

Author: rudkx

lib/SILOptimizer/PassManager/Passes.cpp

@@ -274,7 +274,6 @@ void swift::runSILOptimizationPasses(SILModule &Module) {
   PM.setStageName("HighLevel");
   AddSSAPasses(PM, OptimizationLevelKind::HighLevel);
   PM.runOneIteration();
-  PM.runOneIteration();
   PM.resetAndRemoveTransformations();
 
   PM.setStageName("EarlyLoopOpt");
@@ -294,7 +293,6 @@ void swift::runSILOptimizationPasses(SILModule &Module) {
   PM.setStageName("MidLevel");
   AddSSAPasses(PM, OptimizationLevelKind::MidLevel);
   PM.runOneIteration();
-  PM.runOneIteration();
   PM.resetAndRemoveTransformations();
 
   // Perform lowering optimizations.