Added a workaround for MSVC 12 (VS2013) optimizer bug.

The compiler incorrectly merges unaligned loads with SIMD comparison
instruction, which causes crashes in Release x64 builds.

https://svn.boost.org/trac/boost/ticket/8509#comment:3
https://connect.microsoft.com/VisualStudio/feedbackdetail/view/981648#tabs