[LangRef] *.overflow intrinsics now support vectors

We have all the necessary legalization, expansion and unrolling support required for the *.overflow intrinsics with vector types, so update the docs to make that clear.

Note: vectorization is not in place yet (the non-homogenous return types aren't well supported) so we still must explicitly use the vectors intrinsics and not reply on slp/loop.

Differential Revision: https://reviews.llvm.org/D58618

llvm-svn: 354821