[BOLT][NFCI] Remove redundant instance of MCAsmBackend

[maksfb]

Summary

Use instance of MCAsmBackend from BinaryContext instead of creating a
new one.

bolt/lib/Rewrite/RewriteInstance.cpp

Out = std::make_unique<ToolOutputFile>(opts::OutputFilename, EC,
                                          sys::fs::OF_None);
   check_error(EC, "cannot create output executable file");
 
   raw_fd_ostream &OS = Out->os();
 
   // Copy allocatable part of the input.
   OS << InputFile->getData().substr(0, FirstNonAllocatableOffset);
 
-  // We obtain an asm-specific writer so that we can emit nops in an
-  // architecture-specific way at the end of the function.
-  std::unique_ptr<MCAsmBackend> MAB(
-      BC->TheTarget->createMCAsmBackend(*BC->STI, *BC->MRI, MCTargetOptions()));
   auto Streamer = BC->createStreamer(OS);
   // Make sure output stream has enough reserved space, otherwise
   // pwrite() will fail.
   uint64_t Offset = OS.seek(getFileOffsetForAddress(NextAvailableAddress));
   (void)Offset;
   assert(Offset == getFileOffsetForAddress(NextAvailableAddress) &&
          "error resizing output file");
 
for (BinaryFunction *Function : BC->getAllBinaryFunctions()) {
 
     OS.pwrite(reinterpret_cast<char *>(Function->getImageAddress()),
               Function->getImageSize(), Function->getFileOffset());
 
     // Write nops at the end of the function.
     if (Function->getMaxSize() != std::numeric_limits<uint64_t>::max()) {
       uint64_t Pos = OS.tell();
       OS.seek(Function->getFileOffset() + Function->getImageSize());
-      MAB->writeNopData(OS, Function->getMaxSize() - Function->getImageSize(),
-                        &*BC->STI);
+      BC->MAB->writeNopData(
+          OS, Function->getMaxSize() - Function->getImageSize(), &*BC->STI);
 
       OS.seek(Pos);
     }
 
     if (!Function->isSplit())
       continue;
 
     // Write cold part