isPredTy can be called on ill-kinded types, especially (of course) if
there is a kind error.  We don't wnat it to crash, but it was, in
piResultTy.

This patch introduces piResultTy_maybe, and uses it in isPredTy.

Ugh.  I dislike this code.  It's mainly used to know when we should
print types with '=>', and we should probably have a better way to
signal that.