glsl: Add new expressions for INTEL_shader_integer_functions2

glsl: Add new expressions for INTEL_shader_integer_functions2

v2: Re-write iadd64_saturate and isub64_saturate to avoid undefined
overflow behavior.  Also fix copy-and-paste bug in isub64_saturate.
Suggested by Caio.

v3: Avoid signed integer overflow for abs_sub(0, INT_MIN).  Noticed by
Caio.

v4: Alternate fix for signed integer overflow for abs_sub(0, INT_MIN).
I tried the previous methon in a small test program with -ftrapv, and it
failed.

Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/767>