vc4: Switch to more QPU-instruction-like QIR instructions

[anholt]

In VC5, I moved QPU instruction packing to the very end of the compiler pipeline, and the middle (including QIR) works on an unpacked QPU instruction structure full of enums, plus information on the side for tracking the QIR register files (so we can allocate temps). This has felt really nice for writing compiler code on VC5, and we should probably either port it back to VC4 or try writing a VC4 port in the VC5 compiler.

The first step would be to do QPU pack/unpack/disasm in the style of VC5, with unit tests.