nir/lower_int64: replace uadd_sat with ior for find_lsb64 and ufind_msb64
I forgot to add this when fixing up some vvl errors with zink.
Fixes crashes with the integer_ctz CL CTS tests in zink.
Fixes: 39ec184d ("zink: lower 64 bit find_lsb, ufind_msb and bit_count")