[memprof] Skip MemProfUsePass on the empty module

This patch teaches the MemProfUsePass on the empty module.

Aside from saving time to deserialize the MemProf profile, this patch
ensures that we can obtain TLI like so:

  TargetLibraryInfo &TLI = FAM.getResult<TargetLibraryAnalysis>(*M.begin());

when we undrift the MemProf profile in near future.

Author: kazutakahirata

llvm/lib/Transforms/Instrumentation/MemProfiler.cpp

@@ -1121,6 +1121,10 @@ MemProfUsePass::MemProfUsePass(std::string MemoryProfileFile,
 }
 
 PreservedAnalyses MemProfUsePass::run(Module &M, ModuleAnalysisManager &AM) {
+  // Return immediately if the module doesn't contain any function.
+  if (M.empty())
+    return PreservedAnalyses::all();
+
   LLVM_DEBUG(dbgs() << "Read in memory profile:");
   auto &Ctx = M.getContext();
   auto ReaderOrErr = IndexedInstrProfReader::create(MemoryProfileFileName, *FS);