nir: Make nir_ssa_def_rewrite_uses take an SSA value

This should make all of NIR ever so slightly faster. More importantly, it'll be more ergonomic because we don't have to keep typing nir_src_for_ssa() every time we do a replacement. It also moves us one step closer to getting rid of non-SSA sources and destinations. Unfortunately, it's a flag-day changes but hopefully one that won't be too bad to deal with during rebasing.

The last two patches will be squashed before they're landed. I broke it in two to separate the manual change vs. the bit I did with Coccinelle.