[llvm] [LLVM][TTI][SME] Allow optional auto-vectorisation for streaming functions. (PR #101679)

[Paul Walker via llvm-commits]

paulwalker-arm wrote:

I'm working on a PR to remove TTI::enableScalableVectorization() because the function looks redundant given the maturity of scalable vectors.  That's a bigger change though and I want to cherry-pick this fix into LLVM 19 and so have created this minimal PR.

https://github.com/llvm/llvm-project/pull/101679