[NFC][SimplifyCFG] Add a test where we fail to preserve DomTree validity

test/Transforms/SimplifyCFG/SimplifyTerminatorOnSelect-domtree-preservation-edgecase.ll

@@ -0,0 +1,58 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -S -simplifycfg -simplifycfg-require-and-preserve-domtree=0 < %s | FileCheck %s
+
+declare void @widget()
+declare void @baz(i8)
+declare void @snork()
+declare void @spam(i8)
+declare void @zot()
+
+define void @wombat(i64 %arg, i1 %arg1) {
+; CHECK-LABEL: @wombat(
+; CHECK-NEXT:  bb:
+; CHECK-NEXT:    [[COND:%.*]] = icmp eq i64 [[ARG:%.*]], 0
+; CHECK-NEXT:    br i1 [[COND]], label [[BB4:%.*]], label [[BB2:%.*]]
+; CHECK:       bb2:
+; CHECK-NEXT:    call void @widget()
+; CHECK-NEXT:    br label [[BB4]]
+; CHECK:       bb4:
+; CHECK-NEXT:    [[TMP:%.*]] = phi i8 [ 0, [[BB:%.*]] ], [ 1, [[BB2]] ]
+; CHECK-NEXT:    call void @baz(i8 [[TMP]])
+; CHECK-NEXT:    call void @snork()
+; CHECK-NEXT:    call void @spam(i8 4)
+; CHECK-NEXT:    ret void
+;
+bb:
+  switch i64 %arg, label %bb2 [
+  i64 0, label %bb3
+  ]
+
+bb2:                                              ; preds = %bb
+  call void @widget()
+  br label %bb3
+
+bb3:                                              ; preds = %bb2, %bb
+  %tmp = phi i8 [ 0, %bb ], [ 1, %bb2 ]
+  br label %bb4
+
+bb4:                                              ; preds = %bb3
+  call void @baz(i8 %tmp)
+  %tmp5 = select i1 %arg1, i64 6, i64 3
+  switch i64 %tmp5, label %bb7 [
+  i64 1, label %bb6
+  i64 0, label %bb8
+  ]
+
+bb6:                                              ; preds = %bb4
+  call void @zot()
+  br label %bb8
+
+bb7:                                              ; preds = %bb4
+  call void @snork()
+  br label %bb8
+
+bb8:                                              ; preds = %bb7, %bb6, %bb4
+  %tmp9 = phi i8 [ 2, %bb4 ], [ 3, %bb6 ], [ 4, %bb7 ]
+  call void @spam(i8 %tmp9)
+  ret void
+}