Fix over-zealous unused-import warning

See Note [Un-warnable import decls] in RnNames.

Fixes Trac #9061.

(cherry picked from commit 9f3e39d5f8686e511ffca406a6e056dec4095e53)
Second go at fixing #9061

My first attempt introduce a bug in -fprint-minimal-imports, but
fortunately a regression test caught it.

(cherry picked from commit 5b73dc5fda1941d51827ea72614782c10a355a3d)
Fix a serious, but rare, strictness analyser bug (Trac #9128)

In a special case for trivial RHSs (see DmdAnal.unpackTrivial),
I'd forgotten to include a demand for the RHS itself.
See Note [Remember to demand the function itself].

Thanks to David Terei for guiding me to the bug,
at PLDI in Edinburgh.

(cherry picked from commit 7d9feb264a4fc3c15d1e5f88f2e7a04202ed9743)
In splitHsFunType, take account of prefix (->)

This fixes Trac #9096

(cherry picked from commit 770e16fceee60db0c2f79e3b77f6fc619bc1d864)