[clang] [llvm] [Loads] Check context instruction for context-sensitive derefability (PR #109277)

[Danila Malyutin via cfe-commits]

danilaml wrote:

@nikic Does `AllowEphemerals` actually do what we want in that case? Haven't looked in detail, but on the surface it seemed way more involved than just `if (I == CtxI) return true;`.

https://github.com/llvm/llvm-project/pull/109277