Specifically, if a variable is marked with IgnoreLiveness=true, then:
1. Completely avoid adding any segments to its live range
2. Assert that no one tries to add segments to its live range
This is done in part by incorporating Variable::IgnoreLiveness into Liveness::RangeMask.
Also, change a functor into a lambda because C++11.
BUG= none
R=jvoung@chromium.org
Review URL: https://codereview.chromium.org/1273823003.
