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

[Danila Malyutin via llvm-commits]

danilaml wrote:

@nikic also, another situation the current impl doesn't seem to account for is IR like
```llvm
bb1:
  %p1 = load dereferenceable(N)
...
bb2:
  %p2 = load dereferenceable(M)
...
common:
  %p = phi ptr [%p1, %bb1], [%p2, %bb2]
  %res = load i32 %p ; <-- dereferenceable(min(N,M)) at least
```

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