DAGCombiner: Canonicalize vector integer abs in the same way we do it for scalars.

This already helps SSE2 x86 a lot because it lacks an efficient way to
represent a vector select. The long term goal is to enable the backend to match
a canonicalized pattern into a single instruction (e.g. vabs or pabs).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@180597 91177308-0d34-0410-b5e6-96231b3b80d8