[llvm] [SLP] Fix cost estimation of external uses with wrong VF (PR #148185)

[Gaëtan Bossu via llvm-commits]

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@@ -3898,7 +3898,7 @@ class BoUpSLP {
 
     /// When ReuseReorderShuffleIndices is empty it just returns position of \p
     /// V within vector of Scalars. Otherwise, try to remap on its reuse index.
-    int findLaneForValue(Value *V) const {
+    unsigned findLaneForValue(Value *V) const {
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gbossu wrote:

Note: The reason I'm changing the return type is because the compiler would throw a warning in `assert(EU.Lane < BundleWidth && "Extracted lane out of bounds.")` if comparing signed/unsigned integers.

Internally in `findLaneForValue()`, the lane was anyway computed as an `unsigned` already.

https://github.com/llvm/llvm-project/pull/148185