[llvm] [InstCombine] Canonicalize scalable GEPs to use llvm.vscale intrinsic (PR #90569)

[David Green via llvm-commits]

https://github.com/davemgreen approved this pull request.

I gave this a test and _almost_ nothing changed. So looks good in that regard.

Our lowering through LSR isn't fantastic at the moment, so there is a chance this is going from sub-optimal to sub-optimal, but I think it makes sense to canonicalize them now and wok with that. LGTM from what I've seen.

https://github.com/llvm/llvm-project/pull/90569