nir_opcodes.py: Saturate to expression that doesn't overflow

nir_opcodes.py: Saturate to expression that doesn't overflow

Compiler warns about overflow when assigning UINT64_MAX to something
smaller than a uin64_t:

src/compiler/nir/nir_constant_expressions.c:16909:50: warning: implicit conversion from 'unsigned long long' to 'uint1_t' (aka 'unsigned char') changes value from 18446744073709551615 to 255 [-Wconstant-conversion]
            uint1_t dst = (src0 + src1) < src0 ? UINT64_MAX : (src0 + src1);
                    ~~~                          ^~~~~~~~~~

Shift UINT64_MAX down to the appropriate maximum value for the type
being assigned to.

Signed-off-by: Kristian H. Kristensen <hoegsberg@google.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>