Do not use optimized atomic libcalls for misaligned atomics.

Summary:
The optimized (__atomic_foo_<n>) libcalls assume that the atomic object
is properly aligned, so should never be called on an underaligned
object.

This addresses one of several problems identified in PR38846.

Reviewers: jyknight, t.p.northover

Subscribers: jfb, cfe-commits

Differential Revision: https://reviews.llvm.org/D51817

llvm-svn: 341734