nir/constant_expressions: Rework Boolean handling

nir/constant_expressions: Rework Boolean handling

This commit contains three related changes.  First, we define boolN_t
for N = 8, 16, and 64 and move the definition of boolN_vec to the loop
with the other vec definitions.  Second, there's no reason why we need
the != 0 on the source because that happens implicitly when it's
converted to bool.  Third, for destinations, we use a signed integer
type and just do -(int)bool_val which will give us the 0/-1 behavior we
want and neatly scales to all bit widths.

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Tested-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>