LV: reuse getSmallBestKnownTC in RT-check TC estimation

GeneratedRTChecks::getCost duplicates getSmallBestKnownTC partially,
when attempting to get the best trip-count estimate. Since the intent of
this code is to get the best trip-count estimate, and
getSmallBestKnownTC is written for exactly this purpose, replace the
partial code-duplication with a call to this function.

Author: artagnon

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -1971,14 +1971,9 @@ class GeneratedRTChecks {
           // count. Assume that the outer loop executes at least twice.
           unsigned BestTripCount = 2;
 
-          // If exact trip count is known use that.
-          if (unsigned SmallTC = SE->getSmallConstantTripCount(OuterLoop))
-            BestTripCount = SmallTC;
-          else if (LoopVectorizeWithBlockFrequency) {
-            // Else use profile data if available.
-            if (auto EstimatedTC = getLoopEstimatedTripCount(OuterLoop))
-              BestTripCount = *EstimatedTC;
-          }
+          // Get the best known TC estimate.
+          if (auto EstimatedTC = getSmallBestKnownTC(*SE, OuterLoop))
+            BestTripCount = *EstimatedTC;
 
           BestTripCount = std::max(BestTripCount, 1U);
           InstructionCost NewMemCheckCost = MemCheckCost / BestTripCount;