update_test_checks: add new --filter-out-after option (#129739)

Whilst trying to clean up some loop vectoriser IR tests (see
test/Transforms/LoopVectorize/AArch64/partial-reduce-chained.ll
for example) a reviewer on PR #129047 suggested it would be
nice to have an option to stop generating CHECK lines after a
certain point. Typically when performing a transformation with
the loop vectoriser we don't usually care about any CHECK lines
generated for the scalar tail of the loop, since the scalar
loop is kept intact. Previously if you wanted to eliminate such
unwanted CHECK lines you had to run the update script, then
manually delete all the lines corresponding to the scalar loop.
This can be very time consuming if the tests ever need changing.

What I've tried to do here is add a new --filter-out-after
option alongside the existing --filter* options that provides
support for stopping the generation of any CHECK lines beyond
the line that matches the filter. With the existing filter
options we never generate CHECK-NEXT lines, but we still care
about ordering with --filter-out-after so I've amended the
code to ensure we treat this filter differently.