This CL changes the datastructure to store live variables from a
std::bitset<256> to a std::vector<bool> to support an arbitrary number
of locals. Unfortunately, std::vector<bool> does not define |= and &=
operators, so I added them on the Environment class.

R=vegorov@chromium.org, mstarzinger@chromium.org, machenbach@chromium.org
BUG=v8:5970

Review-Url: https://codereview.chromium.org/2694103005
Cr-Commit-Position: refs/heads/master@{#43216}