StringPiece clean up.

Profile guided clean up.
Try to avoid creating StringPieces with the contents of a dex file where
the length is known.
Try to avoid RegTypeCache::FromDescriptor when there's a class available.
Make ConstantType::ConstantValue inlinable.
Saving of about 50ms from a 2 threaded ThinkFree compile on host.

Change-Id: I47a12c3c76f46e2c9805be1c3a3e3870fe1f5d85