llc: don't call llvm_shutdown twice

[jfb]

Summary

InitLLVM already calls llvm_shutdown, but llc registers for shutdown
with llvm_shutdown_obj so it gets called twice. It's not hurting anything, but
it's also not useful, so don't do it.

[ruiu]

LGTM

tools/llc/llc.cpp

 //
 int main(int argc, char **argv) {
   InitLLVM X(argc, argv);
 
   // Enable debug stream buffering.
   EnableDebugBuffering = true;
 
   LLVMContext Context;
-  llvm_shutdown_obj Y;  // Call llvm_shutdown() on exit.
 
   // Initialize targets first, so that --version shows registered targets.
   InitializeAllTargets();
   InitializeAllTargetMCs();
   InitializeAllAsmPrinters();
   InitializeAllAsmParsers();
 
   // Initialize codegen and IR passes used by llc so that the -print-after,