It's not clear why the code was looking for signed chars < 0, but it can't
matter to the assembler anyway, so the check goes away. This also fixes
compatibility with arrays of [us]byte that have constantexprs in them.
Also slightly restructure some code to be cleaner.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@10854 91177308-0d34-0410-b5e6-96231b3b80d8