[PowerPC, DAGCombiner] Fold a << (b % (sizeof(a) * 8)) back to a single instruction

Summary:
This is the corresponding llvm change to D28037 to ensure no performance
regression.

Reviewers: bogner, kbarton, hfinkel, iteratee, echristo

Subscribers: nemanjai, llvm-commits

Differential Revision: https://reviews.llvm.org/D28329

llvm-svn: 301990