WorkWrap: Handle partial FUN apps in `isRecDataCon` (#21265)

Partial FUN apps like `(->) Bool` aren't detected by `splitFunTy_maybe`.
A silly oversight that is easily fixed by replacing `splitFunTy_maybe` with a
guard in the `splitTyConApp_maybe` case.

But fortunately, Simon nudged me into rewriting the whole `isRecDataCon`
function in a way that makes it much shorter and hence clearer which DataCons
are actually considered as recursive.

Fixes #21265.