Correctly pass backend and target to self-profile query

[Kobzol]

This was causing 502 errors when non-default data was present in one of the examined commits.

Also turns the assertion into a user-visible error, to make this easier to debug.

[Kobzol]

(@lqd this was causing the 502 errors on the detailed page)

[lqd]

Does the link that used to fail work for you with this PR? (Can we even test this locally with the measureme files being on the server/S3?)

[Kobzol]

So, the previous link won't work (the link has to be different, so that we construct the DB query correctly). But this doesn't solve the whole issue - if you try to access a link for a Cranelift result, it will still either fail or return empty data. To fix that I'll have to refactor how we store self-profile data on S3 (will work on that as a follow-up).

This just fixes situations where the data is available, and you access the LLVM result, but there is also some associated Cranelift data.

[lqd]

Ok, and at least it won't pollute master commits from now on, as you also had; that's better either way than just crashing.

[lqd]

🚀