[LV] Check if value was already not uniform for previous VF.

If the value was already known to not be uniform for the previous
(smaller VF), it cannot be uniform for the larger VF.

This slightly reduces compile-time, once uniformity checks are becoming
a bit more expensive due to using SCEV rewriting (D148841).

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D151658

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -4672,9 +4672,17 @@ void LoopVectorizationCostModel::collectLoopUniforms(ElementCount VF) {
   if (Cmp && TheLoop->contains(Cmp) && Cmp->hasOneUse())
     addToWorklistIfAllowed(Cmp);
 
+  auto PrevVF = VF.divideCoefficientBy(2);
   // Return true if all lanes perform the same memory operation, and we can
   // thus chose to execute only one.
   auto isUniformMemOpUse = [&](Instruction *I) {
+    // If the value was already known to not be uniform for the previous
+    // (smaller VF), it cannot be uniform for the larger VF.
+    if (PrevVF.isVector()) {
+      auto Iter = Uniforms.find(PrevVF);
+      if (Iter != Uniforms.end() && !Iter->second.contains(I))
+        return false;
+    }
     if (!Legal->isUniformMemOp(*I, VF))
       return false;
     if (isa<LoadInst>(I))