[UpdateTestChecks] Handle prefix reuse for appended check lines

[jdoerfert]

Summary

When we appended check lines at the end we could not share prefixes
before. This patch should make it possible and allow us to reduce
some check line counts (especially for Clang/OpenMP tests).

See also: https://reviews.llvm.org/D128686

[mtrofin]

nit: in the commit message, there's a spurious 'the' before 'some check counts'

[jhuber6]

LGTM, others may want to comment.