Run simplifier only when the env is clean.
This fixes #10896. In the indexed-types/should_fail/BadSock test, there is a bad type definition. This gets type-checked, an error gets reported, but then **GHC keeps going**. Later, when running the simplifier to do an ambiguity check, the bad type environment causes GHC to fall over. My solution: only run the simplifier in a clean, error-free type environment. A downside of this is that fewer error messages are reported. This makes me a bit sad, but I'm not sure how to avoid the problem. Suggestions welcome.