Have the SubtargetFeature help routine just not return a number and
fall back to the normal path without a cpu. While doing this fix
llc to just exit when we don't have a module to process instead of
asserting.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208102 91177308-0d34-0410-b5e6-96231b3b80d8