zink: lower b2b to b2i

zink: lower b2b to b2i

Zink requires 1-bit booleans, but this requirement was missed before
b2b1s started getting automatically inserted. Let's lower these away, to
avoid piglit regressions.

Fixes the following piglits:
- shaders@glsl-vs-if-bool
- spec@!opengl 2.0@vertex-program-two-side

Fixes: c217ee8d35f ("nir: Insert b2b1s around booleans in nir_lower_to")
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/2902
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4903>