[LV] Use SCEV::getElementCount in selectEpilogueVectorizationFactor. (#150018)

Follow-up to #149789 to use
getElementCount to compute the remaining iterations in
selectEpilogueVectrizationFactor.

PR: #150018

Author: fhahn

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -4500,19 +4500,17 @@ VectorizationFactor LoopVectorizationPlanner::selectEpilogueVectorizationFactor(
   Type *TCType = Legal->getWidestInductionType();
   const SCEV *RemainingIterations = nullptr;
   unsigned MaxTripCount = 0;
-  if (MainLoopVF.isFixed()) {
-    // TODO: extend to support scalable VFs.
-    const SCEV *TC = vputils::getSCEVExprForVPValue(
-        getPlanFor(MainLoopVF).getTripCount(), SE);
-    assert(!isa<SCEVCouldNotCompute>(TC) &&
-           "Trip count SCEV must be computable");
-    RemainingIterations = SE.getURemExpr(
-        TC, SE.getConstant(TCType, MainLoopVF.getFixedValue() * IC));
-
-    // No iterations left to process in the epilogue.
-    if (RemainingIterations->isZero())
-      return Result;
+  const SCEV *TC =
+      vputils::getSCEVExprForVPValue(getPlanFor(MainLoopVF).getTripCount(), SE);
+  assert(!isa<SCEVCouldNotCompute>(TC) && "Trip count SCEV must be computable");
+  RemainingIterations =
+      SE.getURemExpr(TC, SE.getElementCount(TCType, MainLoopVF * IC));
+
+  // No iterations left to process in the epilogue.
+  if (RemainingIterations->isZero())
+    return Result;
 
+  if (MainLoopVF.isFixed()) {
     MaxTripCount = MainLoopVF.getFixedValue() * IC - 1;
     if (SE.isKnownPredicate(CmpInst::ICMP_ULT, RemainingIterations,
                             SE.getConstant(TCType, MaxTripCount))) {