[llvm] [WIP][VPlan] Use BlockFrequencyInfo in getPredBlockCostDivisor (PR #158690)

[Luke Lau via llvm-commits]

================
@@ -435,78 +435,16 @@ define void @test_first_order_recurrence_tried_to_scalarized(ptr %dst, i1 %c, i3
 ; CHECK-LABEL: @test_first_order_recurrence_tried_to_scalarized(
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    [[N:%.*]] = select i1 [[C:%.*]], i32 8, i32 9
-; CHECK-NEXT:    br label [[VECTOR_PH:%.*]]
-; CHECK:       vector.ph:
-; CHECK-NEXT:    [[N_RND_UP:%.*]] = add i32 [[N]], 3
-; CHECK-NEXT:    [[N_MOD_VF:%.*]] = urem i32 [[N_RND_UP]], 4
-; CHECK-NEXT:    [[N_VEC:%.*]] = sub i32 [[N_RND_UP]], [[N_MOD_VF]]
-; CHECK-NEXT:    [[TRIP_COUNT_MINUS_1:%.*]] = sub i32 [[N]], 1
-; CHECK-NEXT:    [[BROADCAST_SPLATINSERT:%.*]] = insertelement <4 x i32> poison, i32 [[TRIP_COUNT_MINUS_1]], i64 0
-; CHECK-NEXT:    [[BROADCAST_SPLAT:%.*]] = shufflevector <4 x i32> [[BROADCAST_SPLATINSERT]], <4 x i32> poison, <4 x i32> zeroinitializer
-; CHECK-NEXT:    br label [[VECTOR_BODY:%.*]]
-; CHECK:       vector.body:
-; CHECK-NEXT:    [[INDEX:%.*]] = phi i32 [ 0, [[VECTOR_PH]] ], [ [[INDEX_NEXT:%.*]], [[PRED_STORE_CONTINUE6:%.*]] ]
-; CHECK-NEXT:    [[VEC_IND:%.*]] = phi <4 x i32> [ <i32 0, i32 1, i32 2, i32 3>, [[VECTOR_PH]] ], [ [[VEC_IND_NEXT:%.*]], [[PRED_STORE_CONTINUE6]] ]
-; CHECK-NEXT:    [[VECTOR_RECUR:%.*]] = phi <4 x i32> [ <i32 poison, i32 poison, i32 poison, i32 4>, [[VECTOR_PH]] ], [ [[VEC_IND]], [[PRED_STORE_CONTINUE6]] ]
-; CHECK-NEXT:    [[TMP0:%.*]] = shufflevector <4 x i32> [[VECTOR_RECUR]], <4 x i32> [[VEC_IND]], <4 x i32> <i32 3, i32 4, i32 5, i32 6>
-; CHECK-NEXT:    [[TMP1:%.*]] = icmp ule <4 x i32> [[VEC_IND]], [[BROADCAST_SPLAT]]
----------------
lukel97 wrote:

Previously blocks that were predicated because of tail folding were discounted by 0.5x by `computePredInstDiscount` because they were technically predicated. However in practice they're always going to be executed except for the last iteration. BFI correctly returns that the probability is ~100% for these instructions which removes the discount.

https://github.com/llvm/llvm-project/pull/158690