[llvm] [LV] Ignore user-specified interleave count when unsafe. (PR #153009)

[Kerry McLaughlin via llvm-commits]

kmclaughlin-arm wrote:

The latest commit puts this PR back to something closer to the original version I posted. I tried to move the legality check into `selectInterleaveCount` (see commit [20fe70](https://github.com/llvm/llvm-project/pull/153009/commits/20fe702e2e84da337152139581ba5bf061b36751)), but as @david-arm commented earlier this means that `selectInterleaveCount` would change the value of `UserIC` after it has been passed to `LVP.plan()`, which does not seem correct.

I'm happy to add a separate function for the legality checks if this would be preferable; the reason for not choosing this route is that I believe `isSafeForAnyVectorWidth` is currently the only condition in `selectInterleaveCount` that we should not allow a user-specified interleave count to override.

https://github.com/llvm/llvm-project/pull/153009