gallivm: add buffer operations to the tgsi->llvm conversion.

gallivm: add buffer operations to the tgsi->llvm conversion.

This adds load, store and atomic operations. These operations
have to respect the exec_mask, and can't operate in lanes where
the execute is off. This is needed to avoid side effects seen
outside the shaders.

There is also bounds checking on the ssbo accesses vs the size
ptr.

Reviewed-by: Roland Scheidegger <sroland@vmware.com>