Adds a boolean flag to the liveness analysis which makes it also analyze
the accumulator. This can help prevent the accumulator escaping loops,
as well as decreasing the number of distinct state values nodes in the
graph.

The flag is a kind of ugly way to hack this in, however it is probably
the simplest to add, and (more importantly) to remove once the AST graph
builder is gone.

I measure a 2.6% improvement on Mandreel on my x64 machine, and a ~2%
improvement on Navier-Stokes. Other improvements are expected.

Review-Url: https://codereview.chromium.org/2428503002
Cr-Commit-Position: refs/heads/master@{#40359}