While I was using the new AccessUtils for a new optimization pass I discovered some areas for improvements. Also I found some bugs.
Changes:
* AccessBase: remove the unhealthy redundancy between `kind` and `baseAddress` types. Now AccessBase is single enum with the relevant base objects/addresses as payloads.
* AccessBase: for `global`, store the `GlobalValue` and not a `global_address` instruction, which is more accurate (because there can be multiple `global_addr`s for a single global variable)
* AccessBase: drop the support for function argument "pointers". The `pointer` is now always a `pointer_to_address` instruction. This also simplifies `PointerIdentification`: either it finds a matching `address_to_pointer` or it bails.
* AccessBase: improve `func isDistinct(from:)`. There are more possibilities to prove that two access bases do not alias.
* AccessBase: replace `var isUniquelyIdentified` with `var hasKnownStorageKind` which is more useful for aliasing checking.
* AccessPath: fix `func isDistinct(from:)`. `SmallProjectionPath.matches` is the wrong way to check if two expression paths may overlap. Instead use the new `SmallProjectionPath.mayOverlap`.
* AccessStoragePathWalker: rename `getAccessStorage` -> `visitAccessStorageRoots` and let it return false if it's not a class/reference AccessBase.
* add tests for `AccessPath.isDistinct(from:)`
