[BasicAA] Don't consider Argument as escape source (NFCI)

The case of an Argument and an identified function local is already
handled earlier, because we don't care about captures in that case.
As such, we don't need to additionally consider the combination of
an Argument with a non-escaping identified function local.

This ensures that isEscapeSource() only returns true for
instructions, which is necessary for D110368.