glsl: Implement uaddCarry() built-in for ARB_gpu_shader5.
glsl: Implement uaddCarry() built-in for ARB_gpu_shader5.

i965 implements this with a single (multiple destination) instruction,
ADDC. Emitting ADDC directly from uaddCarry() would be ideal, but our
optimization passes don't know how to copy with expressions with
side-effects.

Radeon has an ADDC_UINT instruction that only generates the carry
bit. I've chosen to go this route and implement uaddCarry() by doing the
addition and the carry operations separately.

Reviewed-by: Kenneth Graunke <bd2562f754ec92342f93f61c25d731e290a2ffa8@whitecape.org>
Reviewed-by: Ian Romanick <2b237cafb16dc45038e85df6c85e74e6d899eba9@intel.com>

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