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

[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@301990 91177308-0d34-0410-b5e6-96231b3b80d8