[DAGCombine] More diamong carry pattern optimization.

Summary:
This diff improve the capability of DAGCOmbine to generate linear carries propagation in presence of a diamond pattern. It is now able to match a large variety of different patterns rather than some hardcoded one.

Arguably, the codegen in test cases is not better, but this is to be expected. The goal of this transformation is more about canonicalisation than actual optimisation.

Reviewers: hfinkel, RKSimon, craig.topper

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D57302

llvm-svn: 365051