Eliminate redundant seq's (Trac #8900)

This patch makes the simplifier eliminate a redundant seq like
    case x of y -> ...y....
where y is used strictly.  GHC used to do this, but I made it less
aggressive in

   commit 28d9a03253e8fd613667526a170b684f2017d299 (Jan 2013)

However #8900 shows that doing so sometimes loses good
transformations; and the transformation is valid according to "A
semantics for imprecise exceptions".  So I'm restoring the old
behaviour.

See Note [Eliminating redundant seqs]