ART: Refactor scalar loop optimizations.

Refactor scalar loop peeling and unrolling to eliminate repeated
checks and graph traversals, to make the code more readable and
to make it easier to add new scalar loop opts.

This is a prerequisite for full unrolling patch.

Test: 530-checker-peel-unroll.
Test: test-art-target, test-art-host.
Change-Id: If824a95f304033555085eefac7524e59ed540322