[InstCombine][Test] Test for fix of replacing select with Phis when branch has the same labels

An additional test that allows to check the correctness of handling the case of the same
branch labels in the dominator when trying to replace select with phi-node.

Patch By: Kirill Polushin
Differential Revision: https://reviews.llvm.org/D84006
Reviewed By: mkazantsev

Author: xortator

llvm/test/Transforms/InstCombine/select.ll

@@ -2000,6 +2000,21 @@ merge:
   ret i32 %s
 }
 
+define i32 @select_dominating_cond_same_labels(i1 %cond) {
+; CHECK-LABEL: @select_dominating_cond_same_labels(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br i1 false, label [[EXIT:%.*]], label [[EXIT]]
+; CHECK:       exit:
+; CHECK-NEXT:    [[RESULT:%.*]] = select i1 [[COND:%.*]], i32 123, i32 456
+; CHECK-NEXT:    ret i32 [[RESULT]]
+;
+entry:
+  %result = select i1 %cond, i32 123, i32 456
+  br i1 %cond, label %exit, label %exit
+exit:
+  ret i32 %result
+}
+
 define i32 @select_phi_same_condition(i1 %cond, i32 %x, i32 %y, i32 %z) {
 ; CHECK-LABEL: @select_phi_same_condition(
 ; CHECK-NEXT:  entry: