ir_to_mesa: Remove the BURG code.

ir_to_mesa: Remove the BURG code.

The promise of the BURG was to recognize multi-instruction sequences
and emit reduced sequences for them.  It would have worked well for
recognizing MUL+ADD -> MAD and possibly even MIN(MAX(val, 0), 1) ->
MOV_SAT with some grammar changes.  However, that potential benefit in
making those optimizations easy is outweighed by the fragility of
monoburg, the amount of (incorrect, as I wrote it) code for using it,
and the burden it was going to cause for handling operations on
aggregate types.