Refactor globalOrStatementSpecific()

Refactor globalOrStatementSpecific()

Current implementation elicits a nullness warning, which is not quite
there -- at the cost of verifyNotNull().

If we refactor checks we can actually make it more performant and
transparent. This is advanteous bacause a null parent implies current is
a GLOBAL storage -- and we therefore can just check the storage type for
STATEMENT_LOCAL.

As it turns out we can refactor the loop to make it obvious, in that we
manually check parent storage. This makes the code flow nicer and more
transparent -- there are no IDE-invisible non-null guarantees. A null
parent storage indicates GLOBAL storage, hence we can bail.

A further boon is that we can simplify the StorageType check to only
compare to STATEMENT_LOCAL and

Change-Id: I5a5b7faf312db6de7f1cebe654f18e9785f58bac
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>