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/compiler/nir/meson.build

@@ -123,6 +123,7 @@ files_libnir = files(
   'nir_lower_constant_initializers.c',
   'nir_lower_double_ops.c',
   'nir_lower_drawpixels.c',
+  'nir_lower_frexp.c',
   'nir_lower_global_vars_to_local.c',
   'nir_lower_gs_intrinsics.c',
   'nir_lower_load_const_to_scalar.c',