ART: Ignore vreg=-1 in test 913

Compiled code may have references that do not originate from a
dalvik register, e.g., when caching values from LSE. Do not print
these roots to make the test robust wrt/ compiler changes.

Bug: 31385354
Test: m test-art-host-run-test-913-heaps
Change-Id: I804faa4cdbfd7140d0faeab877bd4fe96748a302