[DAGCombine] (addcarry 0, 0, X) -> (ext/trunc X)

[DAGCombine] (addcarry 0, 0, X) -> (ext/trunc X)

Summary:
While this makes some case better and some case worse - so it's unclear if it is a worthy combine just by itself - this is a useful canonicalisation.

As per discussion in D32756 .

Reviewers: jyknight, nemanjai, mkuper, spatel, RKSimon, zvi, bkramer

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D32916

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@303441 91177308-0d34-0410-b5e6-96231b3b80d8