[X86] Use MOVQ for i64 atomic_stores when SSE2 is enabled

Summary: If we have SSE2 we can use a MOVQ to store 64-bits and avoid falling back to a cmpxchg8b loop. If its a seq_cst store we need to insert an mfence after the store.

Reviewers: spatel, RKSimon, reames, jfb, efriedma

Reviewed By: RKSimon

Subscribers: hiraditya, dexonsmith, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60546

llvm-svn: 359368