ART: Introduce Uint8 loads in compiled code.

Some vectorization patterns are not recognized anymore.
This shall be fixed later.

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Test: testrunner.py --target --optimizing on Nexus 5X
Test: Nexus 5X boots.
Bug: 23964345
Bug: 67935418
Change-Id: I587a328d4799529949c86fa8045c6df21e3a8617