nir: Convert to nir_foreach_function_impl

Done by hand at each call site but going very quickly with funny Vim motions and
common regexes. This is a very common idiom in NIR.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23807>

src/compiler/nir/nir_opt_dce.c

@@ -251,8 +251,8 @@ bool
 nir_opt_dce(nir_shader *shader)
 {
    bool progress = false;
-   nir_foreach_function(function, shader) {
-      if (function->impl && nir_opt_dce_impl(function->impl))
+   nir_foreach_function_impl(impl, shader) {
+      if (nir_opt_dce_impl(impl))
          progress = true;
    }