Use some less-known grep flags to avoid the for loop in
check-unused-bailouts.sh, speeding it up considerably.

Before:
    $ time tools/check-unused-bailouts.sh >/dev/null

    real	0m14.717s
    user	0m6.968s
    sys	0m7.128s

After:
    $ time tools/check-unused-bailouts.sh >/dev/null

    real	0m0.514s
    user	0m0.480s
    sys	0m0.028s

Change-Id: I2178e2adaf60282865cf18272246a77bcff9d140
Reviewed-on: https://chromium-review.googlesource.com/597690Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47076}