Simplify the generation of Objective-C setters, at least a little.

Simplify the generation of Objective-C setters, at least a little.
Use a more portable heuristic for deciding when to emit a single
atomic store;  it's possible that I've lost information here, but
I'm not sure how much of the logic before was intentionally arch-specific
and how much was just not quite consistent.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@139468 91177308-0d34-0410-b5e6-96231b3b80d8