Currently NodeList is defined as std::vector<CfgNode*>, but in the future it may be desirable to change it to something like std::list<CfgNode*> so that it is easier to split edges and insert the new nodes at the right locations, rather than re-sorting them in a separate pass.

This gets us closer by using foo.front() instead of foo[0].  There are still a couple more places using the [] operator, but the changes would be more intrusive.

Also, a few instances of ".size()==0" are changed to the possibly more efficient ".empty()".

BUG= none
R=jvoung@chromium.org, kschimpf@google.com

Review URL: https://codereview.chromium.org/704753007