ir_to_mesa: Implement ir_binop_logic_or using an add w/saturate or add w/SLT

ir_to_mesa: Implement ir_binop_logic_or using an add w/saturate or add w/SLT

Logical-or is implemented using addition (followed by clampling to
[0,1]) on values of 0.0 and 1.0.  Replacing the logical-or operators
with addition gives a + b which has a result on the range [0, 2].

Previously a SNE instruction was used to clamp the resulting logic
value to [0,1].  In a fragment shader, using a saturate on the add has
the same effect.  Adding the saturate to the add is free, so (at
least) one instruction is saved.

In a vertex shader, using an SLT on the negation of the add result has
the same effect.  Many older shader architectures do not support the
SNE instruction.  It must be emulated using two SLT instructions and
an ADD.  On these architectures, the single SLT saves two
instructions.

Reviewed-by: Eric Anholt <eric@anholt.net>