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.

Author: mtrofin

llvm/test/CodeGen/AArch64/arm64-homogeneous-prolog-epilog-tail-call.mir

@@ -1,4 +1,5 @@
-# RUN: llc -verify-machineinstrs -mtriple=arm64-applie-ios7.0 -start-before=aarch64-lower-homogeneous-prolog-epilog -homogeneous-prolog-epilog %s
+# RUN: rm -rf %S/arm64-homogeneous-prolog-epilog-tail-call.s
+# RUN: llc -verify-machineinstrs -mtriple=arm64-applie-ios7.0 -start-before=aarch64-lower-homogeneous-prolog-epilog -homogeneous-prolog-epilog %s -o /dev/null
 #
 # This test ensures defined registers are preserved after lowering homogeneous
 # epilog into helper calls. Without the fix, the verifier would complain about