[llvm] [InstCombine] Improve select equiv fold for plain condition (PR #83405)

[via llvm-commits]

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@@ -3759,3 +3759,18 @@ define i32 @sequence_select_with_same_cond_extra_use(i1 %c1, i1 %c2){
   %s3 = select i1 %c1, i32 789, i32 %s2
   ret i32 %s3
 }
+
+define i8 @sequence_select_with_same_cond_multi_arms(i1 %cond0, i1 %cond1, i8 %a, i8 %b) {
+; CHECK-LABEL: @sequence_select_with_same_cond_multi_arms(
+; CHECK-NEXT:    [[SEL2:%.*]] = select i1 [[TMP1:%.*]], i8 [[A:%.*]], i8 [[B:%.*]]
+; CHECK-NEXT:    [[SEL1_NEW:%.*]] = select i1 [[COND1:%.*]], i8 [[A]], i8 2
+; CHECK-NEXT:    [[SEL4:%.*]] = select i1 [[COND1]], i8 [[SEL2]], i8 3
+; CHECK-NEXT:    [[SEL3:%.*]] = select i1 [[TMP1]], i8 [[SEL1_NEW]], i8 [[SEL4]]
+; CHECK-NEXT:    ret i8 [[SEL3]]
+;
+  %sel0 = select i1 %cond0, i8 %a, i8 %b
----------------
vfdff wrote:

* this case  has 2  transformation in combine, https://alive2.llvm.org/ce/z/ChFi5F
  - first
  ```
define i8 @fist(i1 %cond0, i1 %cond1, i8 %a, i8 %b) {
  %sel0 = select i1 %cond0, i8 %a, i8 %b
  %sel1 = select i1 %cond1, i8 %sel0, i8 2
  %sel2 = select i1 %cond1, i8 %sel0, i8 3 ; update directly base on first 3 nodes
  %sel3 = select i1 %cond0, i8 %sel1, i8 %sel2
  ret i8 %sel3
}
  ```
  - second
  ```
define i8 @second(i1 %cond0, i1 %cond1, i8 %a, i8 %b) {
    %sel0 = select i1 %cond0, i8 %a, i8 %b
    %sel1.new = select i1 %cond1, i8 %a, i8 2
    %sel2 = select i1 %cond1, i8 %sel0, i8 3
    %sel3 = select i1 %cond0, i8 %sel1.new, i8 %sel2; base on the 4 nodes
    ret i8 %sel3
  }
  ```

https://github.com/llvm/llvm-project/pull/83405