JIT: Optimized iCache invalidation

There are 2 issues that were addressed:
- VS2013 generates insane code for std::bitset, so this was replaced with std::vector<bool>. It behaves similarly to a bitset, but Microsoft compiler is able to generate reasonable code for it.
- memset-ing iCache to JIT_ICACHE_INVALID_BYTE is a significant cost and, by the looks of things, only happens for sanity checking reasons.

Fixing those issues significantly improves performance in some sections of MGS: Twin Snakes. Other games might be positively affected too.