[DAGCombine] Make sure we check the ResNo from UADDO before combining

Summary: UADDO has 2 result, and one must check the result no before doing any kind of combine. Without it, the transform is invalid.

Reviewers: joerg

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D34088

llvm-svn: 305162