ART: Implement scalar loop peeling.

Implement scalar loop peeling for invariant exits elimination
(on arm64). If the loop exit condition is loop invariant then
loop peeling + GVN + DCE can eliminate this exit in the loop
body. Note: GVN and DCE aren't applied during loop optimizations.

Note: this functionality is turned off by default now.

Test: test-art-host, test-art-target, boot-to-gui.

Change-Id: I98d20054a431838b452dc06bd25c075eb445960c