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

[Danila Malyutin via cfe-commits]

danilaml wrote:

@nikic one thing I've discovered with this fact is that now `load` is not dereferenceable at "load" itself due to `isValidAssumeForContext` quirk of not allowing `assume` affect itself. Not sure if it's intentional for attributes here - makes it look like it's not safe to speculate load in its current position. There is also some friction with instruction vs iterator (i.e. when you only have insertion point at the end of bb so can't easily turn it into an instruction), but that's a different matter.

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