ART: Improve RegType::IsAssignableFrom

Change the chained if-else-if to a switch by introducing an
AssignmentType enum and corresponding getter in RegType.

Reduces instruction count from 39.4B to 39.3B and branch count
from 9.6B to 9.5B when compiling a well-known large app with
the verify compiler filter.

Bug: 10921004
Test: m test-art-host
Change-Id: I2d72df78f281715015829ff555a3c4c2446c03be