Move createEliminateAvailableExternallyPass earlier

[Yaron Keren via cfe-commits]

Following the discussion on cfe-dev, this
moves createEliminateAvailableExternallyPass earlier in the pass pipeline
to save running many ModulePasses on available external functions that are
thrown away anyhow.
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Index: lib/Transforms/IPO/PassManagerBuilder.cpp
===================================================================
--- lib/Transforms/IPO/PassManagerBuilder.cpp	(revision 246535)
+++ lib/Transforms/IPO/PassManagerBuilder.cpp	(working copy)
@@ -331,6 +331,19 @@
   // we must insert a no-op module pass to reset the pass manager.
   MPM.add(createBarrierNoopPass());
 
+  if (!DisableUnitAtATime && OptLevel > 1 && !PrepareForLTO) {
+    // Remove avail extern fns and globals definitions if we aren't
+    // compiling an object file for later LTO. For LTO we want to preserve
+    // these so they are eligible for inlining at link-time. Note if they
+    // are unreferenced they will be removed by GlobalDCE later, so
+    // this only impacts referenced available externally globals.
+    // Eventually they will be suppressed during codegen, but eliminating
+    // here enables more opportunity for GlobalDCE as it may make
+    // globals referenced by available external functions dead
+    // and saves running remaining passes on the eliminated functions.
+    MPM.add(createEliminateAvailableExternallyPass());
+  }
+
   if (EnableNonLTOGlobalsModRef)
     // We add a fresh GlobalsModRef run at this point. This is particularly
     // useful as the above will have inlined, DCE'ed, and function-attr
@@ -438,17 +451,6 @@
     // GlobalOpt already deletes dead functions and globals, at -O2 try a
     // late pass of GlobalDCE.  It is capable of deleting dead cycles.
     if (OptLevel > 1) {
-      if (!PrepareForLTO) {
-        // Remove avail extern fns and globals definitions if we aren't
-        // compiling an object file for later LTO. For LTO we want to preserve
-        // these so they are eligible for inlining at link-time. Note if they
-        // are unreferenced they will be removed by GlobalDCE below, so
-        // this only impacts referenced available externally globals.
-        // Eventually they will be suppressed during codegen, but eliminating
-        // here enables more opportunity for GlobalDCE as it may make
-        // globals referenced by available external functions dead.
-        MPM.add(createEliminateAvailableExternallyPass());
-      }
       MPM.add(createGlobalDCEPass());         // Remove dead fns and globals.
       MPM.add(createConstantMergePass());     // Merge dup global constants
     }