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/intel/vulkan/anv_pipeline.c

@@ -226,6 +226,8 @@ anv_shader_compile_to_nir(struct anv_device *device,
    NIR_PASS_V(nir, nir_lower_io_to_temporaries,
               entry_point->impl, true, false);
 
+   NIR_PASS_V(nir, nir_lower_frexp);
+
    /* Vulkan uses the separate-shader linking model */
    nir->info.separate_shader = true;