nir/search: Don't compare 8-bit or 1-bit constants with floats

nir/search: Don't compare 8-bit or 1-bit constants with floats

Without this, adding an algebraic rule like

   (('bcsel', ('flt', a, 0.0), 0.0, ...), ...),

will cause assertion failures inside nir_src_comp_as_float in
GTF-GL46.gtf21.GL.lessThan.lessThan_vec3_frag (and related tests) from
the OpenGL CTS and shaders/closed/steam/witcher-2/511.shader_test from
shader-db.

All of these cases have some code that ends up like

   ('bcsel', ('flt', a, 0.0), 'b@1', ...)

When the 'b@1' is tested, nir_src_comp_as_float fails because there's
no such thing as a 1-bit float.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Thomas Helland <thomashelland90@gmail.com>