ART: Don't set initial RTI for BoundType if input untyped

ReferenceTypePropagation will create a BoundType with upper bound
[Object, inexact, not null] for each if-not-null branch. The logic
setting its initial RTI will, however, set it straight to Object if
the input is untyped (loop phi or its derivate). This patch changes
the logic to leave the BoundType untyped and set it during fix-point
iteration.

Bug: 26330326
Change-Id: Ic492e2179a4c51f577908e60fbcf70d728b98a6f