This commit introduces a new mode for bytecode_dispatches_report.py
which reports the top sources of dispatches to a given bytecode and
the top destinations of dispatches from the same bytecode.

The bytecode name is passed with --top-dispatches-for-bytecode
(short form: -f), while the number of sources and destinations to
show is controlled with -n.

BUG=v8:4899
LOG=N

Review-Url: https://codereview.chromium.org/1979233002
Cr-Commit-Position: refs/heads/master@{#36284}