R600: Add cmpxchg instruction for evergreen

Refactored the R600_LDS_1A2D class a bit to get it to actually work.

It seemed to be previously unused and broken.

We also have to disable the conversion to the noret variant for now in
R600ISelLowering because the getLDSNoRetOp method only handles 1A1D LDS ops.

Someone can feel free to modify the AMDGPU::getLDSNoRetOp method to
work for more than 1A1D variants of LDS operations. It's being left as a
future TODO for now.

Signed-off-by: Aaron Watry <awatry at gmail.com>
Reviewed-by: Matt Arsenault <matthew.arsenault@amd.com>
llvm-svn: 217596