Revert "[LV] Add option to still enable the legacy cost model. (#99536)"

Summary:
This reverts commit 9ba5244.

Remove the recently added temporary option vectorize-use-legacy-cost-model
as discussed on the PR adding it, now that we branched for 19.x.

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Tasks: 

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Differential Revision: https://phabricator.intern.facebook.com/D60250748

Author: fhahn

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -206,11 +206,6 @@ static cl::opt<unsigned> VectorizeMemoryCheckThreshold(
     "vectorize-memory-check-threshold", cl::init(128), cl::Hidden,
     cl::desc("The maximum allowed number of runtime memory checks"));
 
-static cl::opt<bool> UseLegacyCostModel(
-    "vectorize-use-legacy-cost-model", cl::init(false), cl::Hidden,
-    cl::desc("Use the legacy cost model instead of the VPlan-based cost model. "
-             "This option will be removed in the future."));
-
 // Option prefer-predicate-over-epilogue indicates that an epilogue is undesired,
 // that predication is preferred, and this lists all options. I.e., the
 // vectorizer will try to fold the tail-loop (epilogue) into the vector body
@@ -9976,9 +9971,8 @@ bool LoopVectorizePass::processLoop(Loop *L) {
       InnerLoopUnroller Unroller(L, PSE, LI, DT, TLI, TTI, AC, ORE, IC, &LVL,
                                  &CM, BFI, PSI, Checks);
 
-      VPlan &BestPlan =
-          UseLegacyCostModel ? LVP.getBestPlanFor(VF.Width) : LVP.getBestPlan();
-      assert((UseLegacyCostModel || BestPlan.hasScalarVFOnly()) &&
+      VPlan &BestPlan = LVP.getBestPlan();
+      assert(BestPlan.hasScalarVFOnly() &&
              "VPlan cost model and legacy cost model disagreed");
       LVP.executePlan(VF.Width, IC, BestPlan, Unroller, DT, false);
 
@@ -10095,18 +10089,14 @@ bool LoopVectorizePass::processLoop(Loop *L) {
         if (!MainILV.areSafetyChecksAdded())
           DisableRuntimeUnroll = true;
       } else {
-        ElementCount Width = VF.Width;
-        VPlan &BestPlan =
-            UseLegacyCostModel ? LVP.getBestPlanFor(Width) : LVP.getBestPlan();
-        if (!UseLegacyCostModel) {
-          assert(size(BestPlan.vectorFactors()) == 1 &&
-                 "Plan should have a single VF");
-          Width = *BestPlan.vectorFactors().begin();
-          LLVM_DEBUG(dbgs()
-                     << "VF picked by VPlan cost model: " << Width << "\n");
-          assert(VF.Width == Width &&
-                 "VPlan cost model and legacy cost model disagreed");
-        }
+        VPlan &BestPlan = LVP.getBestPlan();
+        assert(size(BestPlan.vectorFactors()) == 1 &&
+               "Plan should have a single VF");
+        ElementCount Width = *BestPlan.vectorFactors().begin();
+        LLVM_DEBUG(dbgs() << "VF picked by VPlan cost model: " << Width
+                          << "\n");
+        assert(VF.Width == Width &&
+               "VPlan cost model and legacy cost model disagreed");
         InnerLoopVectorizer LB(L, PSE, LI, DT, TLI, TTI, AC, ORE, Width,
                                VF.MinProfitableTripCount, IC, &LVL, &CM, BFI,
                                PSI, Checks);