[GlobalDCE] [VFE] Add a test for incorrect VFE behavior in presence o…

…f null/invalid vtable entries

Add a test for VFE where there's several vtables, and one of them contains an
invalid entry (from VFE's perspective), and which causes VFE to incorrectly skip
scanning subsequent vtables and drop their dependencies.

llvm/test/Transforms/GlobalDCE/virtual-functions-null.ll

@@ -0,0 +1,44 @@
+; RUN: opt < %s -globaldce -S | FileCheck %s
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+
+declare { i8*, i1 } @llvm.type.checked.load(i8*, i32, metadata)
+
+@vtableA = internal unnamed_addr constant { [2 x i8*] } { [2 x i8*] [
+  i8* null,
+  i8* bitcast (void ()* @vfunc2 to i8*)
+]}, align 8, !type !{i64 0, !"vfunc1.type"}, !type !{i64 8, !"vfunc2.type"}, !vcall_visibility !{i64 2}
+
+; CHECK:      @vtableA = internal unnamed_addr constant { [2 x i8*] } { [2 x i8*] [
+; CHECK-SAME:   i8* null,
+; CHECK-SAME:   i8* bitcast (void ()* @vfunc2 to i8*)
+; CHECK-SAME: ] }, align 8
+
+@vtableB = internal unnamed_addr constant { [2 x i8*] } { [2 x i8*] [
+  i8* bitcast (void ()* @vfunc1 to i8*),
+  i8* bitcast (void ()* @vfunc2 to i8*)
+]}, align 8, !type !{i64 0, !"vfunc1.type"}, !type !{i64 8, !"vfunc2.type"}, !vcall_visibility !{i64 2}
+
+; CHECK:      @vtableB = internal unnamed_addr constant { [2 x i8*] } { [2 x i8*] [
+; CHECK-SAME:   i8* null,
+; CHECK-SAME:   i8* bitcast (void ()* @vfunc2 to i8*)
+; CHECK-SAME: ] }, align 8
+
+define internal void @vfunc1() {
+  ret void
+}
+
+define internal void @vfunc2() {
+  ret void
+}
+
+define void @main() {
+  %1 = ptrtoint { [2 x i8*] }* @vtableA to i64 ; to keep @vtableA alive
+  %2 = ptrtoint { [2 x i8*] }* @vtableB to i64 ; to keep @vtableB alive
+  %3 = tail call { i8*, i1 } @llvm.type.checked.load(i8* null, i32 0, metadata !"vfunc1.type")
+  %4 = tail call { i8*, i1 } @llvm.type.checked.load(i8* null, i32 0, metadata !"vfunc2.type")
+  ret void
+}
+
+!999 = !{i32 1, !"Virtual Function Elim", i32 1}
+!llvm.module.flags = !{!999}