[DAGCombiner] Match (add X, X) as (shl X, 1) when detecting rotate.

Summary: The combiner transforms (shl X, 1) into (add X, X).

Reviewers: craig.topper, efriedma, RKSimon, lebedev.ri

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66882

llvm-svn: 370578