Do not to perform RUN line substitution of llc if that's prefixed by a hypnen

(-llc), similarly to the way it was done for clang and llvmc.

This doesn't affect the upstream llvm tests but helps when developing custom
LLVM-based tools and testing them within the LLVM regression framework.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@183994 91177308-0d34-0410-b5e6-96231b3b80d8