dmdAnal: Move handling of datacon strictness to mkWWstr_one

Previously datacon strictness was accounted for when we demand analysed a case
analysis. However, this results in pessimistic demands in some cases. For
instance, consider the program (from T10482)

    data family Bar a
    data instance Bar (a, b) = BarPair !(Bar a) !(Bar b)
    newtype instance Bar Int = Bar Int

    foo :: Bar ((Int, Int), Int) -> Int -> Int
    foo f k =
      case f of
        BarPair x y -> case burble of
                          True -> case x of
                                    BarPair p q -> ...
                          False -> ...

We really should be able to assume that `p` is already evaluated since it came
from a strict field of BarPair.

However, as written the demand analyser can not conclude this since we may end
up in the False branch of the case on `burble` (which places no demand on `x`).
By accounting for the data con strictness later, applied to the demand of the
RHS, we get the strict demand signature we want.

See Note [Add demands for strict constructors] for a more comprehensive
discussion.

Test Plan: Validate

Reviewers: simonpj, osa1, goldfire

Subscribers: rwbarton, carter

GHC Trac Issues: #15696

Differential Revision: https://phabricator.haskell.org/D5226

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