[LV] Disable VPlan-based cost model for 19.x release.

As discussed in  #92555 flip
the default for the option added in
#99536 to true.

This restores the original behavior for the release branch to give the
VPlan-based cost model more time to mature on main.

Author: fhahn

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -206,7 +206,7 @@ static cl::opt<unsigned> VectorizeMemoryCheckThreshold(
     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,
+    "vectorize-use-legacy-cost-model", cl::init(true), cl::Hidden,
     cl::desc("Use the legacy cost model instead of the VPlan-based cost model. "
              "This option will be removed in the future."));
 

llvm/test/Transforms/LoopVectorize/RISCV/riscv-vector-reverse.ll

@@ -135,7 +135,6 @@ define void @vector_reverse_i64(ptr nocapture noundef writeonly %A, ptr nocaptur
 ; CHECK-NEXT:  LV: Interleaving is not beneficial.
 ; CHECK-NEXT:  LV: Found a vectorizable loop (vscale x 4) in <stdin>
 ; CHECK-NEXT:  LEV: Epilogue vectorization is not profitable for this loop
-; CHECK-NEXT:  VF picked by VPlan cost model: vscale x 4
 ; CHECK-NEXT:  Executing best plan with VF=vscale x 4, UF=1
 ; CHECK-NEXT:  VPlan 'Final VPlan for VF={vscale x 4},UF>=1' {
 ; CHECK-NEXT:  Live-in vp<%0> = VF * UF
@@ -339,7 +338,6 @@ define void @vector_reverse_f32(ptr nocapture noundef writeonly %A, ptr nocaptur
 ; CHECK-NEXT:  LV: Interleaving is not beneficial.
 ; CHECK-NEXT:  LV: Found a vectorizable loop (vscale x 4) in <stdin>
 ; CHECK-NEXT:  LEV: Epilogue vectorization is not profitable for this loop
-; CHECK-NEXT:  VF picked by VPlan cost model: vscale x 4
 ; CHECK-NEXT:  Executing best plan with VF=vscale x 4, UF=1
 ; CHECK-NEXT:  VPlan 'Final VPlan for VF={vscale x 4},UF>=1' {
 ; CHECK-NEXT:  Live-in vp<%0> = VF * UF

llvm/test/tools/UpdateTestChecks/update_analyze_test_checks/Inputs/x86-loopvectorize-costmodel.ll.expected

@@ -17,7 +17,6 @@ define void @test() {
 ; CHECK:  LV: Found an estimated cost of 5 for VF 16 For instruction: %v0 = load float, ptr %in0, align 4
 ; CHECK:  LV: Found an estimated cost of 22 for VF 32 For instruction: %v0 = load float, ptr %in0, align 4
 ; CHECK:  LV: Found an estimated cost of 92 for VF 64 For instruction: %v0 = load float, ptr %in0, align 4
-; CHECK:  LV: Found an estimated cost of 1 for VF 1 For instruction: %v0 = load float, ptr %in0, align 4
 ;
 entry:
   br label %for.body