i965/fs: Define logical texture sampling opcodes.

i965/fs: Define logical texture sampling opcodes.

Each logical variant is largely equivalent to the original opcode but
instead of taking a single payload source it expects the arguments
separately as individual sources, like:

 tex_logical dst, coordinates, shadow_c, lod, lod2,
                  sample_index, mcs, sampler, offset,
                  num_coordinate_components, num_grad_components

This patch defines the opcodes and usual instruction boilerplate,
including a placeholder lowering function provided mostly as
documentation for their source registers.

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>