ART: Refactor VerifiedMethod

Only a small percentage of methods have a non-empty safe-cast set.
Change VerifiedMethod to use a pointer to a set.

Reduces native allocations at compile time. For a large well-known
application, VerifiedMethod memory drops from 12.7MB to 6.3MB.

Bug: 38250868
Test: m test-art-host
Change-Id: I6f0128a2cf09a8afb29027b24975aec12054ce2f