For all RUN lines starting with "not", redirect stderr to /dev/null so tests

don't fail when (expected) error output is produced. This fixes 17 tests.

While I was there, I also made all RUN lines of the form "not llvm-as..." a bit
more consistent, they now all redirect stderr and stdout to /dev/null and use
input redirect to read their input.

llvm-svn: 52174