Fix up MCFixup::getAccessVariant to handle unary expressions.

This allows correct relocations to be generated for a symbolic
address that is being adjusted by a negative constant. Since r204294,
such expressions have triggered undefined behavior when LLVM was built
without assertions.

Credit goes to Rafael for this patch; I'm submitting it on his behalf
as he is on vacation this week.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206192 91177308-0d34-0410-b5e6-96231b3b80d8