nir: Clean up some things around the various fcsel opcodes
I have a couple MRs (one soon, and one in the distant future) that build on top of this.
I'm not super attached to Dropped until the change is easier to defend. nir: Rename nir_op_fcsel to nir_op_fcsel_ne
, but it does make things more orthogonal.
Edited by Ian Romanick