ART: Disallow uses of uninitialized references

The following instructions accepted uninitialized reference types
as their arguments:
 - instance-of
 - check-cast
 - throw
 - iput-object (stored value argument)
 - sput-object
 - invoke-* (non-this arguments)

Monitor-enter and monitor-exit are allowed.

Bug: 26594149

Change-Id: I2a4decb1fba274b8969b17bc237ac0fd19b93c80