Proper handling of diamond-like cases in if-conversion

If converter was somewhat careless about "diamond" cases, where there
was no join block, or in other words, where the true/false blocks did
not have analyzable branches. In such cases, it was possible for it to
remove (needed) branches, resulting in a loss of entire basic blocks.

Differential Revision: http://reviews.llvm.org/D16156

llvm-svn: 258310