glsl: Delete LRP_TO_ARITH lowering pass flag.

glsl: Delete LRP_TO_ARITH lowering pass flag.

Tt's kind of a trap---calling do_common_optimization() after
lower_instructions() may cause opt_algebraic() to reintroduce
ir_triop_lrp expressions that were lowered, effectively defeating the
point.  Because of this, nobody uses it.

v2: Delete more code (caught by Ian Romanick).

Cc: "10.1" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Acked-by: Eric Anholt <eric@anholt.net>