[llvm] [CodeGen] Mark mem intrinsic loads and stores as dereferenceable (PR #80184)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> (Having said that, I don't understand the point of the MODereferenceable flag. In IR `derefenceable` metadata is applied to the thing that creates the pointer, so you get UB at that point if it is _not_ dereferenceable. Applying it to the load/store that uses the pointer seems redundant, since they would always give UB anyway if the pointer is not dereferenceable.)

I thought the point was for code motion, which is kind of useless at the use point


https://github.com/llvm/llvm-project/pull/80184