Create an IR optimisation pass manager.

[otrho]

As we add new optimisation passes we need a structured way to call them.

Some potential features:

• Levels of optimisation mimicking -O0, -Os, -O3, etc.
• Pass categories and metadata:
  • Passes which match a category (e.g., constants) could be requested as a group.
  • Passes which 'request' to be run after a certain operation type. E.g., DCE should always be run after instruction combining since new opportunities may have appeared. Simplify CFG after inlining. This would require a fair bit of design though.
• Heuristics for deciding what to run:
  • Optimisations could be re-run until their returns have diminished below a certain point, or compile time performance limits are met.

But initially we just need a nice way to say 'optimise this IR at this level' from Sway core.

[otrho]

I put something into the opt binary (sway-ir/src/bin/opt.rs) which resembles something we might at least start with.

[vaivaswatha]

More TODOs:

1. Allow specifying individual passes on the command line. So if no opt level is specified, but -dce is specified, then only DCE must be run.
2. Printing the IR must also be treated as a "pass" from a command line perspective, allowing us to print the IR at any point in the pipeline.

Passes which 'request' to be run after a certain operation type. E.g., DCE should always be run after instruction combining

3. We need to have two kinds of dependences here. A hard dependence X->Y (Y hard depends on X) means that Y cannot run without X having run. Hard dependences must be allowed only when X (in the example here) is an analysis and not an optimization. Soft dependences (which need not be enforced, for correctness) can be b/w optimizations. As far as I know LLVM doesn't have this "soft dependence". They just schedule the passes as needed. But I think it's a nice thing to have if we design it all well.

[vaivaswatha]

Design discussion: https://github.com/FuelLabs/sway/discussions/3596 (since GH hasn't added an auto-link).