nir: Some metadata improvements

This MR makes a few passes report metadata more correctly and attempts to make it easier with a new nir_metadata_all enum and some better documentation. It's not nearly as big of a rework as !950 (closed) so I'm hoping that maybe it'll actually land.

For core NIR, I've broken the cleanups into two patches. The first one should be entirely mechanical while the second one requires closer reading. Hopefully that makes review a bit easier.

FYI: I Do not intend to land the last patch in the MR. It exists simply to show that, on Intel, I now have everything for which we use NIR_PASS reporting metadata. I do not expect other drivers to fix all their passes before we land the patches to fix the pile of core NIR issues I found.