[mlir][AMDGPU] Add emulation pass for atomics on AMDGPU targets

Not all AMDGPU targets support all atomic operations. For example,
there are not atomic floating-point adds on the gfx10 series. Add a
pass to emulate these operations using a compare-and-swap loop, by
analogy to the generic atomicrmw rewrite in MemrefToLLVM.

This pass is named generally, as in the future we may have a
memref-to-amdgpu that translates constructs like atomicrmw fmax (which
doesn't generally exist in LLVM) to the relevant intrinsics, which may
themselves require emulation.

Since the AMDGPU dialect now has a pass that operates on it, the
dialect's directory structure is reorganized to match other similarly
complex dialects.

The pass should be run before amdgpu-to-rocdl if desired.

This commit also adds f64 support to atomic_fmax.

Depends on D148722

Reviewed By: nirvedhmeshram

Differential Revision: https://reviews.llvm.org/D148724