Revert "[SCEV] Make computeExitLimit more simple and more powerful"

This reverts commit 023c8be90980e0180766196cba86f81608b35d38.

This patch triggers miscompile of zlib on PowerPC platform. Most likely it is
caused by some pre-backend PPC-specific pass, but we don't clearly know the
reason yet. So we temporally revert this patch with intention to return it
once the problem is resolved. See bug 37229 for details.

llvm-svn: 330893