nir: Add b2b opcodes

nir: Add b2b opcodes

These exist to convert between different types of boolean values.  In
particular, we want to use these for uniform and shared memory
operations where we need to convert to a reasonably sized boolean but we
don't care what its format is so we don't want to make the back-end
insert an actual i2b/b2i.  In the case of uniforms, Mesa can tweak the
format of the uniform boolean to whatever the driver wants.  In the case
of shared, every value in a shared variable comes from the shader so
it's already in the right boolean format.

The new boolean conversion opcodes get replaced with mov in
lower_bool_to_int/float32 so the back-end will hopefully never see them.
However, while we're in the middle of optimizing our NIR, they let us
have sensible load_uniform/ubo intrinsics and also have the bit size
conversion.

Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4338>