Provide links to conveniently compare upstream llvm changes between snapshots

[kwk]

I would be nice if a snapshot issue featured a ready to use link that immediately takes you a page that lets you compare the changes

• since the day before: For example llvm/llvm-project@4e44298...21bcd00 compares the changes between October 29th and 30th.
• changes since today's picked commit llvm/llvm-project@21bcd00...main

Then you can browse the files and see what has changed since yesterday.