[LangRef] fptosi and fptoui return poison on overflow.

I think we assume poison, not undef, for certain transforms we
currently do. In any case, we should clarify the language here.

(This sort of conversion is undefined behavior according to the C
and C++ standards. And in practice, hardware implementations handle
overflow inconsistently, so it would be difficult to define the
result here.)

Differential Revision: https://reviews.llvm.org/D47851

llvm-svn: 334326