JIT: JitBaseBlockCache::valid_block is now std::vector<bool>

This was previously a std::bitset, which resulted in generation of suboptimal code by VS2013 compiler.
See https://gist.github.com/kayru/753db9607d680e26194b
JIT: JitBaseBlockCache::InvalidateICache no longer memsets iCache memory to INVALID_BYTE.

This is redundant, since we reset the valid_block bits any way.
However, this was a source of a significant performance issues in some games.
In particular, Metal Gear Solid: Twin Snakes.
removed trailing whitespace
Merge pull request #396 from kayru/opt_icache_invalidation

JIT: Optimized iCache invalidation