treewide: Convert to use nir_foreach_function_impl and nir_foreach_function_with_impl

In conjunction with !23901 (merged), All usage of nir_foreach_function are converted to nir_foreach_function_impl or nir_foreach_function_with_impl when possible.

This is a following work of !23807 (merged)

And indeed it's founds some unsafe code that doesn't check func->impl != NULL and fixed.

Signed-off-by: Yonggang Luo luoyonggang@gmail.com

cc @alyssa @jenatali if possible, I'd like to receive a-b