Adds Cfg::localCse for basic-block local common-subexpression elimination
If we have
    t1 = op b c
    t2 = op b c
This pass will replace future uses of t2 in a basic block by t1.

To enable, use -enable-experimental in O2

BUG=none
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/1997443002 .