glsl_to_tgsi: implement ir_binop_logic_or using an add w/saturate or add w/SLT

glsl_to_tgsi: implement ir_binop_logic_or using an add w/saturate or add w/SLT

Logical-or is implemented using addition (followed by clamping 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.

Note that SNE is still used when integers are used for boolean values, since
there is no such thing as an integer saturate, and older shader architectures
without SNE don't support integers.

This is a port of commit 41f8ffe5e07c to glsl_to_tgsi with integer support
added.