ART: Collect instruction flags in a struct

Use small backing types for the enums, where possible. Then collect
the flags in a InstructionDescriptor struct. This improves locality
for combined lookups, e.g., in the verifier.

Bug: 10921004
Test: m test-art-host
Change-Id: I2925bce59d2f6fa5fcd8da5007e9567562b1b070