[PowerPC][UpdateTestChecks] Remove the extra # when scrubbing loop comments

Summary:
The patch D63957 is to avoid empty string when scrubbing loop comments,
it will replace loop comments to a `#`, that's correct.
But if the line has something else not only loop comments, we will get
a extra `#`.
The patch is to remove the extra `#`.

Reviewed By: jsji

Differential Revision: https://reviews.llvm.org/D77357