"inferattrs" will deduce the attribute, but it will be too late for
many optimizations. Set it ourselves when creating the call.

Differential Revision: http://reviews.llvm.org/D17598

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@267762 91177308-0d34-0410-b5e6-96231b3b80d8