ART: Implement next-line assertions in Checker

Some tests require verifying an exact sequence of lines in the graph
dump. This was already possible by inserting 'CHECK-NOT: {{.*}}'
between the individual lines, but hardly a convenient way of doing so.
This patch introduces a new 'CHECK-NEXT' kind of assertions that
replaces the old method and will become useful for testing assembly.

Change-Id: I1bb951707bda44320166dc7ef828866a6957a113