llc: don't call llvm_shutdown twice

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.

Reviewers: ruiu

Subscribers: aheejin, llvm-commits

Differential Revision: https://reviews.llvm.org/D46788

llvm-svn: 332174