pan/bi: Scalarize the IR
This MR intends to simplify the Bifrost IR in preparation for better register allocation.
- Remove word offsets into vectors in the IR. Instead, add split and collect pseudoinstructions.
- Only allow register preloading at the start of the shader. This avoids the need to do register dataflow analysis in RA.
- Require vectors to have a consistent dimension. Resizing is accomplished with split+collect sequences.
Edited by Alyssa Rosenzweig