st/mesa: Add a GLSL IR to TGSI translator.

st/mesa: Add a GLSL IR to TGSI translator.

It is still a work in progress at this point, but it produces working and
reasonably well-optimized code.

Originally based on ir_to_mesa and st_mesa_to_tgsi, but does not directly use
Mesa IR instructions in TGSI generation, instead generating TGSI from the
intermediate class glsl_to_tgsi_instruction.  It also has new optimization
passes to replace _mesa_optimize_program.