Make the combiner check if shifts are legal before creating them

Summary: Make sure shifts are legal/specified by the legalizerinfo before creating it

Reviewers: qcolombet, dsanders, rovka, t.p.northover

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D39264

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@316602 91177308-0d34-0410-b5e6-96231b3b80d8