[llvm] [LV] Fix cmp-select reduction phi backedge when fold tail. (PR #190752)

[Elvis Wang via llvm-commits]

================
@@ -330,12 +330,12 @@ define i16 @select_decreasing_induction_icmp_table_i16(i16 noundef %val) {
 ; IC4VF4-NEXT:    [[TMP77:%.*]] = select <4 x i1> [[TMP101]], <4 x i16> <i16 7, i16 6, i16 5, i16 4>, <4 x i16> splat (i16 32767)
 ; IC4VF4-NEXT:    [[TMP70:%.*]] = select <4 x i1> [[TMP102]], <4 x i16> <i16 3, i16 2, i16 1, i16 0>, <4 x i16> splat (i16 32767)
 ; IC4VF4-NEXT:    [[TMP71:%.*]] = select <4 x i1> [[TMP103]], <4 x i16> <i16 -1, i16 -2, i16 -3, i16 -4>, <4 x i16> splat (i16 32767)
-; IC4VF4-NEXT:    br label %[[MIDDLE_BLOCK:.*]]
-; IC4VF4:       [[MIDDLE_BLOCK]]:
 ; IC4VF4-NEXT:    [[TMP112:%.*]] = select <4 x i1> splat (i1 true), <4 x i16> [[TMP76]], <4 x i16> splat (i16 32767)
 ; IC4VF4-NEXT:    [[TMP113:%.*]] = select <4 x i1> splat (i1 true), <4 x i16> [[TMP77]], <4 x i16> splat (i16 32767)
 ; IC4VF4-NEXT:    [[TMP114:%.*]] = select <4 x i1> splat (i1 true), <4 x i16> [[TMP70]], <4 x i16> splat (i16 32767)
 ; IC4VF4-NEXT:    [[TMP115:%.*]] = select <4 x i1> zeroinitializer, <4 x i16> [[TMP71]], <4 x i16> splat (i16 32767)
----------------
ElvisWang123 wrote:

These select has a user  (reduction-phi) during licm.
But the use of reduction-phi will be removed since this loop has small trip count (only 1 vector iteration).

https://github.com/llvm/llvm-project/pull/190752