For umul_low (al * bl), zero is returned if the low 16-bits word of either
source is zero.

for imadsh_mix16 (ah * bl << 16 + c), c is returned if either 'ah' or 'bl'
is zero.

A couple of nir_search_helpers are added:

is_upper_half_zero() returns true if the highest word of all components of
an integer NIR alu src are zero.

is_lower_half_zero() returns true if the lowest word of all components of
an integer nir alu src are zero.

Reviewed-by: Eric Anholt <eric@anholt.net>