[llvm] [LV][EVL] Skip tryAddExplicitVectorLength for plans with scalar VF. (PR #125497)

[Alexey Bataev via llvm-commits]

alexey-bataev wrote:

Better to modify setTailFoldingStyles() and add a check to bool EVLIsLegal:
```
bool EVLIsLegal = IsScalableVF && UserIC <= 1 && ...
```
which was removed accidentally

https://github.com/llvm/llvm-project/pull/125497