Refactor the way dump flags are handled
We were being inconsistent about how we tested whether dump flags were enabled; in particular, sometimes we also checked the verbosity, and sometimes we didn't. This lead to oddities such as "ghc -v4" printing an "Asm code" section which didn't contain any code, and "-v4" enabled some parts of "-ddump-deriv" but not others. Now all the tests use dopt, which also takes the verbosity into account as appropriate.
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