Includes the following:
1. Liveness analysis.
2. Linear-scan register allocation.
3. Address mode optimization.
4. Compare-branch fusing.

All of these depend on liveness analysis.  There are three versions of liveness analysis (in order of increasing cost):
1. Lightweight.  This computes last-uses for variables local to a single basic block.
2. Full.  This computes last-uses for all variables based on global dataflow analysis.
3. Full live ranges.  This computes all last-uses, plus calculates the live range intervals in terms of instruction numbers.  (The live ranges are needed for register allocation.)

For testing the full live range computation, Cfg::validateLiveness() checks every Variable of every Inst and verifies that the current Inst is contained within the Variable's live range.

The cross tests are run with O2 in addition to Om1.

Some of the lit tests (for what good they do) are updated with O2 code sequences.

BUG= none
R=jvoung@chromium.org

Review URL: https://codereview.chromium.org/300563003