[llvm] [InstCombine] Simplify nonnull pointers (PR #128111)

[via llvm-commits]

asmok-g wrote:

Here's a reduction attempt of the whole preprocessed (might be over-reduced):
```
namespace {
template <bool, class a> using b = a;
template <int c> struct ab {
  static const int aa = c;
};
template <class d, class e> struct f : ab<__is_convertible(d, e)> {};
template <int> struct g {
  template <class, class h, class... i>
  using j = g<!h::aa>::template j<h, i...>;
};
template <> struct g<false> {
  template <class k, class> using j = k;
};
template <class... l> using m = g<sizeof...(l)>::template j<ab<false>, l...>;
template <class n, class a> struct ac : m<f<n, a>, int> {};
template <class a> struct o {
  typedef a p;
  p *q;
  o() : q() {}
  p *operator->() { return q; }
};
template <class a> struct r {
  a *q;
  int *s;
  template <class n, b<ac<n, a>::aa, int> = 0> r(o<n>);
  o<a> ad() {
    o<a> t;
    if (s)
      t.q = q;
    return t;
  }
};
} // namespace
struct ae {
  int u[];
};
struct v {
  virtual ae y();
};
o<v> af;
struct z : v {
  z() : w(af) {}
  ae y() {
    o x = w.ad();
    return x->y();
  }
  r<v> w;
};
void ag() { z(); }
```

cmd: `clang  -fno-exceptions   -O3  '-std=gnu++20'  reduced.ii -emit-llvm -S  -o `

[YutongZhuu](https://github.com/YutongZhuu) can you please take another look?

https://github.com/llvm/llvm-project/pull/128111