Add simple MLIR SLP vectorizer

[Hardcode84]

Based on upstream PR (https://github.com/llvm/llvm-project/pull/140469)

Simple SLP vectorizer to combine sequence of scalar ops and smaller vector ops into larger vector ops. Doesn't have any cost model (yet?) therefore "greedy". See code comments for impl details. Mostly adding it to test it in Wave but if you have a good place to plug it in IREE itself we can test it there as well.

[benvanik]

Let's get feedback from the codegen team - if we don't think we'll be using this we'll not want to land it. /cc @MaheshRavishankar (when you're back)

[qedawkins]

This warrants a VC for sure. The core idea with structured op codegen is to not have to do this. I could see a world where we don't mind adding this, but it'll be really important to avoid relying on it as a crutch.

[Hardcode84]

(I didn't even undraft it yet)