[MSSA] Extending IsGuaranteedLoopInvariant to support an instruction defined in the entry block

As mentioned in [[ https://reviews.llvm.org/D96979 | D96979 ]], I'm extending the **IsGuaranteedLoopInvariant** check also to the `MemorySSA.cpp` file.

@fhahn For now I didn't unify the function into `MemorySSA.h` because, as you mentioned, it's not directly MSSA related. I'm open to suggestions to find a better place so we can improve the unification process.

Reviewed By: fhahn

Differential Revision: https://reviews.llvm.org/D97155