[ObjC][ARC] Ignore lifetime markers between *ReturnValue calls

When eliminating a pair of

`llvm.objc.autoreleaseReturnValue`

followed by

`llvm.objc.retainAutoreleasedReturnValue`

we need to make sure that the instructions in between are safe to
ignore.

Other than bitcasts and useless GEPs, it's also safe to ignore lifetime
markers for both static allocas (lifetime.start/lifetime.end) and dynamic
allocas (stacksave/stackrestore).

These get added by the inliner as part of the return sequence and can
prevent the transformation from happening in practice.

Differential Revision: https://reviews.llvm.org/D69833