Util: Improve update_llc_test_checks to scrub macosx-style assembly annotations

Summary:
In D37523 Sanjay pointed out that the tool does not scrub macosx-style 'End of Function' annotations,
where the comments begin with a double-#.

I tested this patch by verifying all existing occurences of 'End function' are scrubbed:
find ./test/CodeGen/X86 -name '*.ll' | xargs grep -l "End function" | xargs utils/update_llc_test_checks.py --llc-binary build/bin/llc

Reviewers: spatel, chandlerc, craig.topper

Reviewed By: spatel

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D37532

llvm-svn: 312678