Split it into three phases: initialization, widening and narrowing.
During widening and narrowing phi-ranges change according to classical widening and narrowing operators defined as:
Widening:
[_|_, _|_] v [a, b] = [a, b]
[a, b] v [c, d] = [c < a ? -inf : a, d > b ? +inf : b]
Narrowing:
[a, b] ^ [c, d] = [(a == -inf) ? c : min(a, c), (b == +inf) ? d : max(b, d)]
R=fschneider@google.com
Review URL: https://codereview.chromium.org//10972003
git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@12772 260f80e4-7a28-3924-810f-c04153c831b5
vegorov@google.com