ac/nir: replace SI.buffer.load.dword with amdgcn.buffer.load

ac/nir: replace SI.buffer.load.dword with amdgcn.buffer.load

The old one generates useless instructions in there, found while
comparing geometry shaders between RadeonSI and RADV.

This improves all Vulkan demos that use geometry shaders, +4%
for deferredshadows, +9% for viewportarray, +7% for
geometryshader on Polaris10.

This seems to also improve DOW3 a little bit (+1%).

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by:  Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>