Fix test outputting to test dir (#171255)
The test introduced in #171118 has `llc` inadvertently producing an
output into the same dir as the test file itself. Most build bots don't
clean up the local git repo, which is assumed to not be written by build
+ test, and patch on top (for build performance reasons), which means
the produced output from the aforementioned PR is treated as a test from
here onwards, by all bots. Since it's missing `RUN` lines, we get
errors, for example
https://lab.llvm.org/buildbot/#/builders/108/builds/20674
This patch fixes the `llc` line and also removes the `.s`. This avoids
all bot maintainers go restart their bots. Then, the cleanup is removed
in #171256.
