nir: port fp16 casting code from dxil

This moves the dxil pass to common code and makes dxil
use the new code.

Acked-by: Adam Jackson <ajax@redhat.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9643>

src/compiler/nir/meson.build

@@ -147,6 +147,7 @@ files_libnir = files(
   'nir_lower_fb_read.c',
   'nir_lower_flatshade.c',
   'nir_lower_flrp.c',
+  'nir_lower_fp16_conv.c',
   'nir_lower_fragcoord_wtrans.c',
   'nir_lower_fragcolor.c',
   'nir_lower_frexp.c',