nir: Rely on the fact that bcsel takes a well formed boolean.

nir: Rely on the fact that bcsel takes a well formed boolean.

According to Connor, it's safe to assume that the first operand of
bcsel, as well as the operand of b2f and b2i, must be well formed
booleans.

https://lists.freedesktop.org/archives/mesa-dev/2016-August/125658.html

With the previous improvements to a@bool handling, this now has no
change in shader-db instruction counts on Broadwell.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Matt Turner <mattst88@gmail.com>