Various improvements in finding induction variables.

Rationale:
(1) Analyze multi-way phis (requested by Nicolas, Igor, and Mingyao).
(2) Analyze trip count for restricted != loops
(3) Added unit test for public API of range analysis (static methods
    were already well-tested).

Change-Id: I9285d22d3bb927f141204cc4697ea6fe5120994d