[llvm] [LV] Split checking if tail-folding is possible, collecting masked ops. (PR #77612)

[Florian Hahn via llvm-commits]

fhahn wrote:

Updated `llvm/test/Transforms/LoopVectorize/RISCV/vectorize-force-tail-with-evl-interleave.ll` to show the impact of the PR:  before this patch, `prepareToFoldTailByMasking` is called before setting the tail-folding decision, so we mark blocks as masked, even if the decision later is to not fold the tail. With the patch, legality check is decoupled from marking blocks as predicated. In the changed test, we now can form an interleave group for scalable vectors, which in turn makes that decision more profitable than the fixed-width VF.

https://github.com/llvm/llvm-project/pull/77612