[llvm] [LoopVectorizer] Prune VFs based on plan register pressure (PR #132190)

[Sam Tebbs via llvm-commits]

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@@ -4885,8 +4872,248 @@ void LoopVectorizationCostModel::collectElementTypesForWidening() {
   }
 }
 
+/// Estimate the register usage for \p Plan and vectorization factors in \p VFs.
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SamTebbs33 wrote:

I had to move it when I was using `LoopVectorizationCostModel` to check for in loop reductions, but now that I'm using the `isInLoop()` function from the reduction recipe we don't need these changes. Reverted.

https://github.com/llvm/llvm-project/pull/132190