[tools] Speed-up check-unused-bailouts.sh

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/597690
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47076}

tools/check-unused-bailouts.sh

@@ -4,13 +4,22 @@
 # found in the LICENSE file.
 
 v8_root=$(readlink -f $(dirname $BASH_SOURCE)/../)
-bailouts=$(grep -oP 'V\(\K(k[^,]*)' "$v8_root/src/bailout-reason.h")
+bailouts=$(
+    grep \
+        --only-matching \
+        --perl-regexp 'V\(\K(k[^,]*)' \
+        -- "$v8_root/src/bailout-reason.h")
 
-for bailout in $bailouts; do
-  bailout_uses=$(grep -r $bailout "$v8_root/src" "$v8_root/test/cctest" | wc -l)
-  if [ $bailout_uses -eq "1" ]; then
-    echo "Bailout reason \"$bailout\" seems to be unused."
-  fi
-done
+# Find bailouts which appear exactly once (in bailout-reason.h)
+grep \
+    --only-matching \
+    --no-filename \
+    --recursive \
+    --word-regexp \
+    --fixed-strings "$bailouts" \
+    -- "$v8_root/src" "$v8_root/test/cctest" \
+| sort \
+| uniq -u \
+| sed -e 's/.*/Bailout reason "&" seems to be unused./'
 
 echo "Kthxbye."