SCRUB_LOOP_COMMENT_RE was introduced in https://reviews.llvm.org/D31285
This works for some loops.
However, we may generate lines with loop comments only.
And since we don't scrub leading white spaces, this will leave an empty
line there, and FileCheck will complain it.
error: found empty check string with prefix 'CHECK:'
This prevented us from using the update_llc_test_checks.py for quite some cases.
We should still keep the comment token there, so that we can safely
scrub the loop comment without breaking FileCheck.