[LangRef] *.overflow intrinsics now support vectors

[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@354821 91177308-0d34-0410-b5e6-96231b3b80d8