GlobalOpt: do not promote globals used atomically to constants.

GlobalOpt: do not promote globals used atomically to constants.

Some atomic loads are implemented as cmpxchg (particularly if large or
floating), and that usually requires write access to the memory involved
or it will segfault.

We can still propagate the constant value to users we understand though.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@360662 91177308-0d34-0410-b5e6-96231b3b80d8