JIT: get rid of useless code in JitCache.cpp.
JitCache: use a custom bit-set class.

Unfortunately, this appears to be necessary for the sake of performance;
the standard library equivalents don't work well enough on Visual Studio
2013. vector<bool>::insert() is way too slow in debug
builds to be usable, and std::bitset generates inefficient code in release
builds.
Merge pull request #404 from magumagu/jitcache-cleanup

JitCache cleanup