[SLP] Harmonise findLaneForValue() return type (NFC) (#148232)

The lane is computed as an unsigned, so let's return it as unsigned.

Author: gbossu

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

@@ -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 {
       unsigned FoundLane = getVectorFactor();
       for (auto *It = find(Scalars, V), *End = Scalars.end(); It != End;
            std::advance(It, 1)) {
@@ -4344,7 +4344,7 @@ class BoUpSLP {
 
   /// This POD struct describes one external user in the vectorized tree.
   struct ExternalUser {
-    ExternalUser(Value *S, llvm::User *U, const TreeEntry &E, int L)
+    ExternalUser(Value *S, llvm::User *U, const TreeEntry &E, unsigned L)
         : Scalar(S), User(U), E(E), Lane(L) {}
 
     /// Which scalar in our function.
@@ -4357,7 +4357,7 @@ class BoUpSLP {
     const TreeEntry &E;
 
     /// Which lane does the scalar belong to.
-    int Lane;
+    unsigned Lane;
   };
   using UserList = SmallVector<ExternalUser, 16>;
 
@@ -7901,7 +7901,7 @@ void BoUpSLP::buildExternalUses(
       // Check if the scalar is externally used as an extra arg.
       const auto ExtI = ExternallyUsedValues.find(Scalar);
       if (ExtI != ExternallyUsedValues.end()) {
-        int FoundLane = Entry->findLaneForValue(Scalar);
+        unsigned FoundLane = Entry->findLaneForValue(Scalar);
         LLVM_DEBUG(dbgs() << "SLP: Need to extract: Extra arg from lane "
                           << FoundLane << " from " << *Scalar << ".\n");
         ScalarToExtUses.try_emplace(Scalar, ExternalUses.size());
@@ -7949,7 +7949,7 @@ void BoUpSLP::buildExternalUses(
 
         if (U && Scalar->hasNUsesOrMore(UsesLimit))
           U = nullptr;
-        int FoundLane = Entry->findLaneForValue(Scalar);
+        unsigned FoundLane = Entry->findLaneForValue(Scalar);
         LLVM_DEBUG(dbgs() << "SLP: Need to extract:" << *UserInst
                           << " from lane " << FoundLane << " from " << *Scalar
                           << ".\n");