LowerAtomic: Don't skip optnone functions; atomic still need lowering (PR34020)

The lowering isn't really an optimization, so optnone shouldn't make a
difference. ARM relies on the pass running when using "-mthread-model
single", because in that mode, it doesn't run AtomicExpand. See bug for
more details.

Differential Revision: https://reviews.llvm.org/D37040

llvm-svn: 311565