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

[Nikita Popov via cfe-commits]

nikic wrote:

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

I expect that the main producer of scalable GEPs is LoopVectorize, which will already use the llvm.vscale representation anyway. So there's probably not many `<vscale x` GEPs in the first place...

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