[llvm] [Instructions] cache computed shufflevector properties (PR #115536)

[Nikita Popov via llvm-commits]

https://github.com/nikic commented:

It may be cleaner to separate out the scalable case into a separate code path, because there is only a single supported scalable shufflevector operation (which is the zero index splat).

https://github.com/llvm/llvm-project/pull/115536