Only have one path in the CFG for ternaries if the condition is known.

The return type analysis requires that the CFG is simplified when the
truth values of branches are statically known at analysis time.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@215114 91177308-0d34-0410-b5e6-96231b3b80d8