[llvm] [LV] Rework collectUnitStridePredicates (PR #216362)

[Ramkumar Ramachandra via llvm-commits]

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@@ -68,20 +68,57 @@ define i64 @predicated_iv_with_liveout(ptr %dst, i64 %n) {
 ; THRESHOLD0-LABEL: define i64 @predicated_iv_with_liveout(
 ; THRESHOLD0-SAME: ptr [[DST:%.*]], i64 [[N:%.*]]) {
 ; THRESHOLD0-NEXT:  [[ENTRY:.*]]:
+; THRESHOLD0-NEXT:    [[TMP0:%.*]] = call i64 @llvm.smax.i64(i64 [[N]], i64 1)
+; THRESHOLD0-NEXT:    [[MIN_ITERS_CHECK:%.*]] = icmp ult i64 [[TMP0]], 4
+; THRESHOLD0-NEXT:    br i1 [[MIN_ITERS_CHECK]], label %[[SCALAR_PH:.*]], label %[[VECTOR_SCEVCHECK:.*]]
+; THRESHOLD0:       [[VECTOR_SCEVCHECK]]:
+; THRESHOLD0-NEXT:    [[SMAX:%.*]] = call i64 @llvm.smax.i64(i64 [[N]], i64 1)
----------------
artagnon wrote:

Not sure how -vectorize-scev-check-threshold is supposed to operate, but isn't this the existing behavior in the unchanged tests as well?

```llvm
; THRESHOLD-LABEL: define i64 @for_liveout_blocked_by_ind_predicate(
; THRESHOLD-SAME: ptr [[DST:%.*]], i64 [[N:%.*]]) {
; THRESHOLD-NEXT:  [[ENTRY:.*]]:
; THRESHOLD-NEXT:    [[TMP0:%.*]] = call i64 @llvm.smax.i64(i64 [[N]], i64 1)
; THRESHOLD-NEXT:    [[MIN_ITERS_CHECK:%.*]] = icmp ult i64 [[TMP0]], 4
; THRESHOLD-NEXT:    br i1 [[MIN_ITERS_CHECK]], label %[[SCALAR_PH:.*]], label %[[VECTOR_PH:.*]]
```

https://github.com/llvm/llvm-project/pull/216362