[NFC][ARM] Regenerate arm64-atomic.ll test

Pre-requisite for D101163, the `NOLSE-0O` case shows registers being
spilled inside the rmw loop.

Use two separate prefixes for the `LSE-O0` case as some outputs differ
only by a comment that update_llc_test_checks.py ignores but lit does
not, causing the test to fail unexpectedly when run.