Don't shrink atomic ops to bool in GlobalOpt.

LLVM IR doesn't currently allow atomic bool load/store operations, and the
transformation is dubious anyway because it isn't profitable on all platforms.

PR17163.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@190357 91177308-0d34-0410-b5e6-96231b3b80d8