[llvm] Fix regular expression to guard against a repeating version string (PR #84575)

[Alexander Richardson via llvm-commits]

arichardson wrote:

Would it be possible to write a LLVM IR test for update_test_checks.py with pre-existing metadata? Or do we only scrub it for update_cc_test_checks.py?

https://github.com/llvm/llvm-project/pull/84575