[AArch64] Use LateSimplifyCFG after expanding atomic operations.

Summary:
After r308422 we defer optimizations that can destroy loop canonical forms to
LateSimplifyCFG. Running LateSimplifyCFG after expanding atomic operations
can exploit more control-flow opportunities.

Reviewers: mcrosier, t.p.northover, efriedma

Reviewed By: efriedma

Subscribers: aemerson, rengolin, javed.absar, llvm-commits, kristof.beyls

Differential Revision: https://reviews.llvm.org/D38262

llvm-svn: 314857