[Sparc] Fix "Cannot select" error for AtomicFence on 32-bit V9

Summary:
This also adds testing of 32-bit V9 atomic lowering, splitting the
64-bit-only tests out into their own file.

Reviewers: venkatra, jyknight

Reviewed By: jyknight

Subscribers: hiraditya, fedor.sergeev, jfb, llvm-commits, glaubitz

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D69352