spirv,nir: lower frexp_exp/frexp_sig inside a new NIR pass

This lowering isn't needed for RADV because AMDGCN has two
instructions. It will be disabled for RADV in an upcoming series.

While we are at it, factorize a little bit.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>

src/amd/vulkan/radv_shader.c

@@ -305,6 +305,7 @@ radv_shader_compile_to_nir(struct radv_device *device,
 
 		NIR_PASS_V(nir, nir_lower_system_values);
 		NIR_PASS_V(nir, nir_lower_clip_cull_distance_arrays);
+		NIR_PASS_V(nir, nir_lower_frexp);
 	}
 
 	/* Vulkan uses the separate-shader linking model */