Consider removing --no-spirt (and outdated codepaths) in Rust-GPU 0.8.

[eddyb]

Timeline so far:

• spirt 0.1.0 behind --spirt: Minimally integrate SPIR-T (opt-in via RUSTGPU_CODEGEN_ARGS=--spirt). #940
• --spirt on by default (--no-spirt opts out): Set SPIR-T as the default IR linker framework #999
• spirt 0.2.0: Integrate the SPIR-T qptr experiment. #1020

(you can read a lot more in each of those PRs)

The only blocker I can think of, for removing --no-spirt, is an external one:

• Naga does not support our "SPIR-V conditional backedges" lifting for loops. spirt#9
  • upstream issue: [spv-in] Conditional loop backedge not lowered to Statement::Loop with break_if expression gfx-rs/naga#1977
  • upstream fix: [spv-in] Convert conditional backedges to break if. gfx-rs/naga#2290

I would prefer to move over to SPIR-T by default, and not maintain two codepaths (especially with experiments like SPIR-T qptr themselves being opt-in, testing all the configurations is getting cumbersome).

Also, if we fully commit to SPIR-T, it should be possible to move debugging/testing features to SPIR-T, to avoid wasteful steps like re-parsing SPIR-V emitted by spirt::spv::lift back into rspirv (which can be also be lossier around e.g. OpLine), etc.