Add a monstrous hack to improve X86ISelDAGToDAG compile time.

 - Force NDEBUG on in any Release build. This drops the compile time to ~100s
   from ~600s, in Release mode.

 - This may just be a temporary workaround, I don't know the true nature of the
   gcc-4.2 compile time performance problem.

llvm-svn: 86695