[UpdateTestChecks][PowerPC] Avoid empty string when scrubbing loop comments

[UpdateTestChecks][PowerPC] Avoid empty string when scrubbing loop comments

Summary:
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.

eg: llvm/test/CodeGen/PowerPC/PR35812-neg-cmpxchg.ll:27:15:
error: found empty check string with prefix 'CHECK:'
; CHECK-NEXT:

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.

Reviewers: timshen, hfinkel, lebedev.ri, RKSimon

Subscribers: nemanjai, jfb, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D63957

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@364775 91177308-0d34-0410-b5e6-96231b3b80d8