[VPlan] Fix assert in store-user in narrowToSingleScalars

Follow up on c2d4c7c ([VPlan] Permit more users in
narrowToSingleScalars) to fix an assert related to WidenStore users of
the recipe being narrowed in narrowToSingleScalars.

Author: artagnon

llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp

@@ -1437,6 +1437,7 @@ static void narrowToSingleScalarRecipes(VPlan &Plan) {
               // we checked the RepOrWidenR operand against
               // vputils::isSingleScalar.
               assert(RepOrWidenR == Store->getAddr() ||
+                     RepOrWidenR == Store->getMask() ||
                      vputils::isSingleScalar(Store->getStoredValue()));
               return true;
             }

llvm/test/Transforms/LoopVectorize/X86/narrow-to-single-scalar.ll

@@ -1,12 +1,33 @@
-; REQUIRES: asserts
-; RUN: not --crash opt -p loop-vectorize -mcpu=skylake -S %s
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 6
+; RUN: opt -p loop-vectorize -mcpu=skylake -S %s | FileCheck %s
 
 target triple = "x86_64-unknown-linux-gnu"
 
 @p = external global [3952 x i8], align 8
 @q = external global [3952 x i8], align 8
 
 define void @narrow_store_user_mask_operand(i32 %x) {
+; CHECK-LABEL: define void @narrow_store_user_mask_operand(
+; CHECK-SAME: i32 [[X:%.*]]) #[[ATTR0:[0-9]+]] {
+; CHECK-NEXT:  [[ENTRY:.*]]:
+; CHECK-NEXT:    br label %[[LOOP_PH:.*]]
+; CHECK:       [[LOOP_PH]]:
+; CHECK-NEXT:    [[IV:%.*]] = phi i64 [ 0, %[[ENTRY]] ], [ [[IV_NEXT:%.*]], %[[LOOP_TAIL:.*]] ]
+; CHECK-NEXT:    [[X_POS:%.*]] = icmp sgt i32 [[X]], 0
+; CHECK-NEXT:    br i1 [[X_POS]], label %[[LOOP_BODY:.*]], label %[[LOOP_TAIL]]
+; CHECK:       [[LOOP_BODY]]:
+; CHECK-NEXT:    [[LD_P:%.*]] = load double, ptr @p, align 8
+; CHECK-NEXT:    [[GEP_Q_IV:%.*]] = getelementptr double, ptr @q, i64 [[IV]]
+; CHECK-NEXT:    [[GEP_Q_IV_8:%.*]] = getelementptr i8, ptr [[GEP_Q_IV]], i64 -8
+; CHECK-NEXT:    store double [[LD_P]], ptr [[GEP_Q_IV_8]], align 8
+; CHECK-NEXT:    br label %[[LOOP_TAIL]]
+; CHECK:       [[LOOP_TAIL]]:
+; CHECK-NEXT:    [[IV_NEXT]] = add i64 [[IV]], 1
+; CHECK-NEXT:    [[EC:%.*]] = icmp eq i64 [[IV]], 1
+; CHECK-NEXT:    br i1 [[EC]], label %[[EXIT:.*]], label %[[LOOP_PH]]
+; CHECK:       [[EXIT]]:
+; CHECK-NEXT:    ret void
+;
 entry:
   br label %loop.ph