Revert r314579: "Recommi r314561 after fixing over-debug assertion".

And follow-up r314585.
Leads to segfaults. I'll forward reproduction instructions to the patch
author.

Also, for a recommit, still add the original patch description.
Otherwise, it becomes really tedious to find out what a patch actually
does. The fact that it is a recommit with a fix is somewhat secondary.

llvm-svn: 314622