LV: reuse getSmallBestKnownTC in a TC estimation (NFC) (#105834)

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

@@ -408,10 +408,11 @@ static bool hasIrregularType(Type *Ty, const DataLayout &DL) {
 /// the following procedure:
 ///   1) Returns exact trip count if it is known.
 ///   2) Returns expected trip count according to profile data if any.
-///   3) Returns upper bound estimate if it is known.
+///   3) Returns upper bound estimate if known, and if \p CanUseConstantMax.
 ///   4) Returns std::nullopt if all of the above failed.
-static std::optional<unsigned> getSmallBestKnownTC(ScalarEvolution &SE,
-                                                   Loop *L) {
+static std::optional<unsigned>
+getSmallBestKnownTC(ScalarEvolution &SE, Loop *L,
+                    bool CanUseConstantMax = true) {
   // Check if exact trip count is known.
   if (unsigned ExpectedTC = SE.getSmallConstantTripCount(L))
     return ExpectedTC;
@@ -421,6 +422,9 @@ static std::optional<unsigned> getSmallBestKnownTC(ScalarEvolution &SE,
     if (auto EstimatedTC = getLoopEstimatedTripCount(L))
       return *EstimatedTC;
 
+  if (!CanUseConstantMax)
+    return std::nullopt;
+
   // Check if upper bound estimate is known.
   if (unsigned ExpectedTC = SE.getSmallConstantMaxTripCount(L))
     return ExpectedTC;
@@ -1933,14 +1937,10 @@ 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, /* CanUseConstantMax = */ false))
+            BestTripCount = *EstimatedTC;
 
           BestTripCount = std::max(BestTripCount, 1U);
           InstructionCost NewMemCheckCost = MemCheckCost / BestTripCount;