ART: Introduce Uint8 compiler data type.

This CL adds all the necessary codegen for the Uint8 type
but does not add code transformations that use that code.
Vectorization codegens are modified to use Uint8 as the
packed type when appropriate. The side effects are now
disconnected from the instruction's type after the graph has
been built to allow changing HArrayGet/H*FieldGet/HVecLoad
to use a type different from the underlying field or array.

Note: HArrayGet for String.charAt() is modified to have
no side effects whatsoever; Strings are immutable.

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing --jit
Test: testrunner.py --target --optimizing on Nexus 6P
Test: Nexus 6P boots.
Bug: 23964345
Change-Id: If2dfffedcfb1f50db24570a1e9bd517b3f17bfd0