Complete unrolling of loops with small body and trip count one.

Rationale:
Avoids the unnecessary loop control overhead, suspend check,
and exposes more opportunities for constant folding in the
resulting loop body. Fully unrolls loop in execute() of
the Dhrystone benchmark (3% to 8% improvements).

Test: test-art-host

Change-Id: If30f38caea9e9f87a929df041dfb7ed1c227aba3