Subzero: Don't store std::string objects inside Variable.

Instead, extend 668a7a333d6ddc3102b5b7c7c07448f6d259113c to include both CfgNode and Variable, keeping a pool of strings in the Cfg.

BUG= none
R=jvoung@chromium.org

Review URL: https://codereview.chromium.org/798693003
11 files changed