short-circuiting conditionals for expressions with potential side-effects.

Conservatively assume aggreate and selection operators have side effects for now.

BUG=
ANGLEBUG=486
R=geofflang@chromium.org, kbr@chromium.org, nicolas@transgaming.com, shannonwoods@chromium.org

Review URL: https://codereview.appspot.com/14441075