Don't recheck obligations if we have learned nothing new

This is just the most trivial check: If no inference variables have been
updated, and there are no new obligations, we can just skip trying to
solve them again. We could be smarter about it, but this already helps
quite a bit, and I don't want to touch this too much before we replace
the inference table by Chalk's.

Fixes #8263 (well, improves it quite a bit).

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