[VPlan] Return std::nullopt early if plans are empty.

Fixes a crash caused by abf5969.

Author: fhahn

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -7296,6 +7296,8 @@ LoopVectorizationPlanner::plan(ElementCount UserVF, unsigned UserIC) {
   buildVPlansWithVPRecipes(ElementCount::getScalable(1), MaxFactors.ScalableVF);
 
   LLVM_DEBUG(printPlans(dbgs()));
+  if (VPlans.empty())
+    return std::nullopt;
   if (all_of(VPlans,
              [](std::unique_ptr<VPlan> &P) { return P->hasScalarVFOnly(); }))
     return VectorizationFactor::Disabled();

llvm/test/Transforms/LoopVectorize/AArch64/cost-no-valid-vplans-built.ll

@@ -0,0 +1,43 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
+; RUN: opt -p loop-vectorize -S %s | FileCheck %s
+
+target datalayout = "e-m:o-i64:64-i128:128-n32:64-S128-Fn32"
+target triple = "arm64-apple-macosx15.0.0"
+
+define void @test(i32 %N, ptr %dst) {
+; CHECK-LABEL: define void @test(
+; CHECK-SAME: i32 [[N:%.*]], ptr [[DST:%.*]]) {
+; CHECK-NEXT:  [[ENTRY:.*]]:
+; CHECK-NEXT:    br label %[[LOOP:.*]]
+; CHECK:       [[LOOP]]:
+; CHECK-NEXT:    [[IV:%.*]] = phi i32 [ 0, %[[ENTRY]] ], [ [[IV_NEXT:%.*]], %[[LOOP]] ]
+; CHECK-NEXT:    [[FNEG137:%.*]] = phi float [ 0.000000e+00, %[[ENTRY]] ], [ [[FNEG1:%.*]], %[[LOOP]] ]
+; CHECK-NEXT:    [[FNEG46:%.*]] = phi float [ 0.000000e+00, %[[ENTRY]] ], [ [[FNEG:%.*]], %[[LOOP]] ]
+; CHECK-NEXT:    [[FNEG]] = fneg float [[FNEG137]]
+; CHECK-NEXT:    [[FNEG1]] = fneg float [[FNEG46]]
+; CHECK-NEXT:    [[GEP:%.*]] = getelementptr float, ptr [[DST]], i32 [[IV]]
+; CHECK-NEXT:    store float [[FNEG1]], ptr [[GEP]], align 4
+; CHECK-NEXT:    [[IV_NEXT]] = add i32 [[IV]], 1
+; CHECK-NEXT:    [[EC:%.*]] = icmp eq i32 [[IV]], [[N]]
+; CHECK-NEXT:    br i1 [[EC]], label %[[EXIT:.*]], label %[[LOOP]]
+; CHECK:       [[EXIT]]:
+; CHECK-NEXT:    ret void
+;
+entry:
+  br label %loop
+
+loop:
+  %iv = phi i32 [ 0, %entry ], [ %iv.next, %loop ]
+  %fneg137 = phi float [ 0.000000e+00, %entry ], [ %fneg1, %loop ]
+  %fneg46 = phi float [ 0.000000e+00, %entry ], [ %fneg, %loop ]
+  %fneg = fneg float %fneg137
+  %fneg1 = fneg float %fneg46
+  %gep = getelementptr float, ptr %dst, i32 %iv
+  store float %fneg1, ptr %gep
+  %iv.next = add i32 %iv, 1
+  %ec = icmp eq i32 %iv, %N
+  br i1 %ec, label %exit, label %loop
+
+exit:
+  ret void
+}