Unrolling and dynamic loop peeling framework in vectorizer.

Rationale:
This CL introduces the basic framework for dynamically peeling
(to obtain aligned access) and unrolling the vector loop (to reduce
looping overhead and allow more target specific optimizations
on e.g. SIMD loads and stores).

NOTE:
The current heuristics are "bogus" and merely meant to exercise
the new framework. This CL focuses on introducing correct code for
the vectorizer. Heuristics and the memory computations for alignment
are to be implemented later.

Test: test-art-target, test-art-host

Change-Id: I010af1475f42f92fd1daa6a967d7a85922beace8