read from dead alloc should return poison

[nunoplopes]

This should not verify:

define i8 @src() {
  %p = alloca i8
  call void @llvm.lifetime.start.p0(ptr %p)
  call void @llvm.lifetime.end.p0(ptr %p)
  %v = load i8, ptr %p
  ret i8 %v
}

define i8 @tgt() {
  unreachable
}

https://alive2.llvm.org/ce/z/GkoNoq

per this LangRef change: llvm/llvm-project#157852