Fix an alignment error in `llvm::expandAtomicRMWToCmpXchg` without breaking...

Fix an alignment error in `llvm::expandAtomicRMWToCmpXchg` without breaking the build where X86 isn't enabled.

Summary: Divide the primitive size in bits by eight so the initial load's alignment is in bytes as expected. Tested with the included unit test.

Reviewers: rengolin, jfb

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D11804

llvm-svn: 244229