Add EP_CGSCCOptimizerLate extension point to PassManagerBuilder

[GorNishanov]

Summary

EP_CGSCCOptimizerLate extension point allows adding CallGraphSCC passes at the end of the main CallGraphSCC passes and before any function simplification passes run by CGPassManager.

Upcoming patch: https://reviews.llvm.org/D22847 utilizes this extensions point.

[GorNishanov]

patch reuploaded with more context (-U999999)

[mehdi_amini]

LGTM.
(This extension point looks quite arbitrary, but not that much more than another...)

llvm/trunk/include/llvm/Transforms/IPO/PassManagerBuilder.h

enum ExtensionPointTy {
     /// should not be disabled by O0 optimization level. The passes will be
     /// inserted after the inlining pass.
     EP_EnabledOnOptLevel0,
 
     /// EP_Peephole - This extension point allows adding passes that perform
     /// peephole optimizations similar to the instruction combiner. These passes
     /// will be inserted after each instance of the instruction combiner pass.
     EP_Peephole,
+
+    /// EP_CGSCCOptimizerLate - This extension point allows adding CallGraphSCC
+    /// passes at the end of the main CallGraphSCC passes and before any
+    /// function simplification passes run by CGPassManager.
+    EP_CGSCCOptimizerLate,
   };
 
   /// The Optimization Level - Specify the basic optimization level.
   ///    0 = -O0, 1 = -O1, 2 = -O2, 3 = -O3
   unsigned OptLevel;
 
   /// SizeLevel - How much we're optimizing for size.
   ///    0 = none, 1 = -Os, 2 = -Oz

llvm/trunk/lib/Transforms/IPO/PassManagerBuilder.cpp

if (Inliner) {
     MPM.add(Inliner);
     Inliner = nullptr;
   }
   if (!DisableUnitAtATime)
     MPM.add(createPostOrderFunctionAttrsLegacyPass());
   if (OptLevel > 2)
     MPM.add(createArgumentPromotionPass()); // Scalarize uninlined fn args
 
+  addExtensionsToPM(EP_CGSCCOptimizerLate, MPM);
   addFunctionSimplificationPasses(MPM);
 
   // FIXME: This is a HACK! The inliner pass above implicitly creates a CGSCC
   // pass manager that we are specifically trying to avoid. To prevent this
   // we must insert a no-op module pass to reset the pass manager.
   MPM.add(createBarrierNoopPass());
 
   if (!DisableUnitAtATime && OptLevel > 1 && !PrepareForLTO &&