glsl: Eliminate ir_assignment::condition

This is something that I have wanted to do, literally, for years. Today I started pulling on a tiny thread, and here I am.

Back in the early days of the GLSL compiler, we had an idea that any assignment could be conditional. The thinking was that this would make it easy to generate predicated instructions on Intel GPUs. For a variety of reasons, this never panned out. Once ir_triop_csel was added to GLSL IR, conditional assignments had no real use.

This MR finally removes all of that from GLSL IR.