i965/fs: Implement lowering of logical texturing opcodes on Gen7+.

i965/fs: Implement lowering of logical texturing opcodes on Gen7+.

This should be largely equivalent to emit_texture_gen7() except that
we now get i965 sampling opcodes directly rather than
ir_texture_opcode enum values.  The mapping is as follows:

 - ir_tex -> SHADER_OPCODE_TEX
 - ir_txb -> FS_OPCODE_TXB
 - ir_txl -> SHADER_OPCODE_TXL
 - ir_txd -> SHADER_OPCODE_TXD
 - ir_txf -> SHADER_OPCODE_TXF
 - ir_txf_ms -> SHADER_OPCODE_TXF_CMS
 - ir_txs -> SHADER_OPCODE_TXS
 - ir_query_levels -> SHADER_OPCODE_TXS too, the visitor will make
                      sure that the provided lod value is zero in this
                      case.
 - ir_lod -> SHADER_OPCODE_LOD
 - ir_tg4 -> SHADER_OPCODE_TG4_OFFSET if the offset value is not
             immediate, SHADER_OPCODE_TG4 otherwise.

Other than that there are only minor changes and style fixes like the
implementation now being factored out in static functions to improve
encapsulation.

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