Move some CLI utils out of llvm-isel-fuzzer and into the library

FuzzMutate might not be the best place for these, but it makes more
sense than an entirely new library for now. This will make setting up
fuzz targets with consistent CLI handling easier.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@312425 91177308-0d34-0410-b5e6-96231b3b80d8
6 files changed