nir: Add nir_shader_intrinsics_pass helper

Like instructions_pass but specialized to intrinsics. More ergnomic for this extremely common case, and possibly a bit faster by avoiding the extra function call on non-intrinsics.

Suggested by @zmike

I got bored converting existing code; as soon as the deletions > insertions I called it a day. I'm sure @jekstrand could whip up a Coccinelle patch to do a flag day change across the tree (including all the drivers' custom NIR passes).