Revert "Renable LLVM's MergeFunctions pass"

This reverts commit r32421.

It broke the incremental bot.

Swift SVN r32422

Author: aschwaighofer

lib/IRGen/IRGen.cpp

@@ -149,7 +149,6 @@ static bool performLLVM(IRGenOptions &Opts, DiagnosticEngine &Diags,
     PMBuilder.Inliner = llvm::createFunctionInliningPass(200);
     PMBuilder.SLPVectorize = false;
     PMBuilder.LoopVectorize = true;
-    PMBuilder.MergeFunctions = true;
   } else {
     PMBuilder.OptLevel = 0;
     if (!Opts.DisableLLVMOptzns)