Renable MergeFunctions

All known bugs have been fixed.

rdar://20920907

Author: aschwaighofer

lib/IRGen/IRGen.cpp

@@ -127,6 +127,7 @@ void swift::performLLVMOptimizations(IRGenOptions &Opts, llvm::Module *Module,
     PMBuilder.Inliner = llvm::createFunctionInliningPass(200);
     PMBuilder.SLPVectorize = true;
     PMBuilder.LoopVectorize = true;
+    PMBuilder.MergeFunctions = true;
   } else {
     PMBuilder.OptLevel = 0;
     if (!Opts.DisableLLVMOptzns)