[WPD] Update llvm.public.type.test after importing functions

I noticed that we are converting llvm.public.type.test to regular
llvm.type.test too early, and thus not updating those in imported
functions. This would result in losing out on WPD opportunities. Move
the update to after function importing, and improve test to cover this
case.

Differential Revision: https://reviews.llvm.org/D150326